made a few methods with descriptive names to wrap that payManaCost with 5 parameters

corrected the extra mana calculation for Nether Void and Spelltithe Enforcer

res/cardsfolder/n/nether_void.txt

@@ -4,7 +4,7 @@ Types:World Enchantment
 T:Mode$ SpellCast | ValidCard$ Card | ValidActivatingPlayer$ Player | TriggerZones$ Battlefield | Execute$ TrigCounter | TriggerDescription$ Whenever a player casts a spell, counter it unless its controller pays 3.
 SVar:TrigCounter:DB$ Counter | Cost$ 0 | UnlessCost$ 3 | Defined$ TriggeredSpellAbility | UnlessPayer$ TriggeredCardController
 SVar:RemRandomDeck:True
-SVar:SpellsNeedExtraMana:3
+SVar:AI_SpellsNeedExtraMana:3
 SVar:Picture:http://www.wizards.com/global/images/magic/general/nether_void.jpg
 Oracle:Whenever a player casts a spell, counter it unless its controller pays {3}.
 SetInfo:LEG Rare

res/cardsfolder/s/spelltithe_enforcer.txt

@@ -4,7 +4,7 @@ Types:Creature Elephant Wizard
 PT:3/3
 T:Mode$ SpellCast | ValidCard$ Card | ValidActivatingPlayer$ Opponent | TriggerZones$ Battlefield | Execute$ TrigSac | TriggerDescription$ Whenever an opponent casts a spell, that player sacrifices a permanent unless he or she pays {1}.
 SVar:TrigSac:DB$ Sacrifice | Defined$ TriggeredCardController | SacValid$ Permanent | SacMessage$ Permanent | UnlessCost$ 1 | UnlessPayer$ TriggeredCardController
-SVar:SpellsNeedExtraMana:1
+SVar:AI_SpellsNeedExtraMana:1 Opponent
 SVar:Picture:http://www.wizards.com/global/images/magic/general/spelltithe_enforcer.jpg
 Oracle:Whenever an opponent casts a spell, that player sacrifices a permanent unless he or she pays {1}.
 SetInfo:GPT Rare

src/main/java/forge/card/ability/ai/ChangeZoneAi.java

@@ -489,7 +489,6 @@ public class ChangeZoneAi extends SpellAbilityAi {
      * @return Card
      */
     private static Card chooseCreature(final Player ai, List<Card> list) {
-        Card card = null;
         Combat combat = new Combat();
         combat.initiatePossibleDefenders(ai);
         List<Card> attackers = ai.getOpponent().getCreaturesInPlay();
@@ -504,16 +503,14 @@ public class ChangeZoneAi extends SpellAbilityAi {
             for (Card c : list) {
                 SpellAbility spell = c.getFirstSpellAbility();
                 spell.setActivatingPlayer(ai);
-               if (ComputerUtilMana.payManaCost(spell, ai, true, 0, false)) {
+                if (ComputerUtilMana.hasEnoughManaSourcesToCast(spell, ai))
-                   card = c;
+                   return c;
-                   break;
             }
+            return null;
         }
-        } else {
+
         // not urgent, get the largest creature possible
-            card = ComputerUtilCard.getBestCreatureAI(list);
+        return ComputerUtilCard.getBestCreatureAI(list);
-        }
-        return card;
     }
 
     // *************************************************************************************
@@ -865,7 +862,7 @@ public class ChangeZoneAi extends SpellAbilityAi {
                             System.out.println("5 Life or less, trying to find something castable.");
                             CardLists.sortByCmcDesc(nonLands);
                             for (Card potentialCard : nonLands) {
-                               if (ComputerUtilMana.payManaCost(potentialCard.getFirstSpellAbility(), ai, true, 0, false)) {
+                               if (ComputerUtilMana.hasEnoughManaSourcesToCast(potentialCard.getFirstSpellAbility(), ai)) {
                                    choice = potentialCard;
                                    break;
                                }
@@ -976,7 +973,7 @@ public class ChangeZoneAi extends SpellAbilityAi {
                             System.out.println("5 Life or less, trying to find something castable.");
                             CardLists.sortByCmcDesc(nonLands);
                             for (Card potentialCard : nonLands) {
-                               if (ComputerUtilMana.payManaCost(potentialCard.getFirstSpellAbility(), ai, true, 0, false)) {
+                               if (ComputerUtilMana.hasEnoughManaSourcesToCast(potentialCard.getFirstSpellAbility(), ai)) {
                                    choice = potentialCard;
                                    break;
                                }
@@ -1210,7 +1207,7 @@ public class ChangeZoneAi extends SpellAbilityAi {
                         for (Card cardInHand : hand) {
                             final SpellAbility spell = cardInHand.getFirstSpellAbility();
                             spell.setActivatingPlayer(ai);
-                            canCastSomething |= ComputerUtilMana.payManaCost(spell, ai, true, 0, false);
+                            canCastSomething |= ComputerUtilMana.hasEnoughManaSourcesToCast(spell, ai);
                         }
                         if (!canCastSomething) {
                             System.out.println("Pulling a land as there are none in hand, less than 4 on the board, and nothing in hand is castable.");
@@ -1229,7 +1226,7 @@ public class ChangeZoneAi extends SpellAbilityAi {
                             System.out.println("5 Life or less, trying to find something castable.");
                             CardLists.sortByCmcDesc(fetchList);
                             for (Card potentialCard : fetchList) {
-                               if (ComputerUtilMana.payManaCost(potentialCard.getFirstSpellAbility(), ai, true, 0, false)) {
+                               if (ComputerUtilMana.hasEnoughManaSourcesToCast(potentialCard.getFirstSpellAbility(), ai)) {
                                    c = potentialCard;
                                    break;
                                }

src/main/java/forge/game/ai/ComputerUtilCost.java

@@ -23,6 +23,7 @@ import forge.card.spellability.SpellAbility;
 import forge.game.player.Player;
 import forge.game.zone.ZoneType;
 import forge.util.MyRandom;
+import forge.util.TextUtil;
 
 /** 
  * TODO: Write javadoc for this type.
@@ -346,10 +347,14 @@ public class ComputerUtilCost {
         // Check for stuff like Nether Void
         int extraManaNeeded = 0;
         if (sa instanceof Spell) {
-            for (Player opp : player.getOpponents()) {
+            for (Card c : player.getGame().getCardsIn(ZoneType.Battlefield)) {
-                for (Card c : opp.getCardsIn(ZoneType.Battlefield)) {
+                final String snem = c.getSVar("AI_SpellsNeedExtraMana");
-                    final String snem = c.getSVar("SpellsNeedExtraMana");
                 if (!StringUtils.isBlank(snem)) {
+                    String[] parts = TextUtil.split(snem, ' ');
+                    boolean meetsRestriction = parts.length == 1 || player.isValid(parts[1], c.getController(), c);
+                    if(!meetsRestriction)
+                        continue;
+
                     try {
                         extraManaNeeded += Integer.parseInt(snem);
                     } catch (final NumberFormatException e) {
@@ -358,13 +363,9 @@ public class ComputerUtilCost {
                 }
             }
         }
-        }
 
-        if (!ComputerUtilMana.payManaCost(sa, player, true, extraManaNeeded, true)) {
+        return ComputerUtilMana.canPayManaCost(sa, player, extraManaNeeded) 
-            return false;
+            && CostPayment.canPayAdditionalCosts(sa.getPayCosts(), sa);
-        }
-    
-        return CostPayment.canPayAdditionalCosts(sa.getPayCosts(), sa);
     } // canPayCost()
 
     public static boolean willPayUnlessCost(SpellAbility sa, Player payer, SpellAbility ability, boolean alreadyPaid, List<Player> payers) {

src/main/java/forge/game/ai/ComputerUtilMana.java

@@ -8,8 +8,6 @@ import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Set;
 
-import org.apache.commons.lang3.StringUtils;
-
 import com.google.common.base.Predicate;
 import forge.Card;
 import forge.CardLists;
@@ -44,6 +42,19 @@ import forge.util.maps.TreeMapOfLists;
  */
 public class ComputerUtilMana {
 
+    public static boolean canPayManaCost(final SpellAbility sa, final Player ai, final int extraMana) {
+        return payManaCost(sa, ai, true, extraMana, true);
+    }
+
+    // Does not check if mana sources can be used right now, just checks for potential chance.
+    public static boolean hasEnoughManaSourcesToCast(final SpellAbility sa, final Player ai) {
+        return payManaCost(sa, ai, true, 0, false);
+    }    
+    
+    public static boolean payManaCost(final Player ai, final SpellAbility sa) {
+        return payManaCost(sa, ai, false, 0, true);
+    }
+
     /**
      * <p>
      * payManaCost.
@@ -62,7 +73,7 @@ public class ComputerUtilMana {
      * @return a boolean.
      * @since 1.0.15
      */
-    public static boolean payManaCost(final SpellAbility sa, final Player ai, final boolean test, final int extraMana, boolean checkPlayable) {
+    private static boolean payManaCost(final SpellAbility sa, final Player ai, final boolean test, final int extraMana, boolean checkPlayable) {
         ManaCostBeingPaid cost = ComputerUtilMana.calculateManaCost(sa, test, extraMana);
         
         final Card card = sa.getSourceCard();
@@ -363,18 +374,6 @@ public class ComputerUtilMana {
 
     }
     
-    /**
-     * <p>
-     * payManaCost.
-     * </p>
-     * 
-     * @param sa
-     *            a {@link forge.card.spellability.SpellAbility} object.
-     */
-    public static boolean payManaCost(final Player ai, final SpellAbility sa) {
-        return payManaCost(sa, ai, false, 0, true);
-    }
-
     /**
      * Find all mana sources.
      * @param manaAbilityMap
@@ -634,17 +633,11 @@ public class ComputerUtilMana {
      * @since 1.0.15
      */
     public static int determineLeftoverMana(final SpellAbility sa, final Player player) {
+        for (int i = 1; i < 100; i++)
+            if (!canPayManaCost(sa, player, i))
+                return i - 1;
 
-        int xMana = 0;
+        return 99;
-    
-        for (int i = 1; i < 99; i++) {
-            if (!payManaCost(sa, player, true, i, true)) {
-                break;
-            }
-            xMana = i;
-        }
-    
-        return xMana;
     }
 
     // Returns basic mana abilities plus "reflected mana" abilities
@@ -655,7 +648,7 @@ public class ComputerUtilMana {
      * 
      * @return a {@link java.util.ArrayList} object.
      */
-    public static final ArrayList<SpellAbility> getAIPlayableMana(Card c) {
+    private static final ArrayList<SpellAbility> getAIPlayableMana(Card c) {
         final ArrayList<SpellAbility> res = new ArrayList<SpellAbility>();
         for (final SpellAbility a : c.getManaAbility()) {