Fixes #987 and #299 (untested)

Example usage:
claim: 50*{arg}

Core/src/main/java/com/intellectualcrafters/plot/commands/Auto.java

@@ -2,6 +2,7 @@ package com.intellectualcrafters.plot.commands;
 
 import com.intellectualcrafters.plot.config.C;
 import com.intellectualcrafters.plot.config.Settings;
+import com.intellectualcrafters.plot.object.Expression;
 import com.intellectualcrafters.plot.object.Plot;
 import com.intellectualcrafters.plot.object.PlotArea;
 import com.intellectualcrafters.plot.object.PlotId;
@@ -116,7 +117,8 @@ public class Auto extends SubCommand {
             }
         }
         if (EconHandler.manager != null && plotarea.USE_ECONOMY) {
-            double cost = plotarea.PRICES.get("claim");
+            Expression<Double> costExp = plotarea.PRICES.get("claim");
+            double cost = costExp.evalute((double) currentPlots);
             cost = (size_x * size_z) * cost;
             if (cost > 0d) {
                 if (EconHandler.manager.getMoney(player) < cost) {

Core/src/main/java/com/intellectualcrafters/plot/commands/Claim.java

@@ -2,6 +2,7 @@ package com.intellectualcrafters.plot.commands;
 
 import com.intellectualcrafters.plot.config.C;
 import com.intellectualcrafters.plot.config.Settings;
+import com.intellectualcrafters.plot.object.Expression;
 import com.intellectualcrafters.plot.object.Location;
 import com.intellectualcrafters.plot.object.Plot;
 import com.intellectualcrafters.plot.object.PlotArea;
@@ -48,7 +49,8 @@ public class Claim extends SubCommand {
         }
         PlotArea world = plot.getArea();
         if ((EconHandler.manager != null) && world.USE_ECONOMY) {
-            double cost = world.PRICES.get("claim");
+            Expression<Double> costExr = world.PRICES.get("claim");
+            double cost = costExr.evalute((double) currentPlots);
             if (cost > 0d) {
                 if (EconHandler.manager.getMoney(player) < cost) {
                     return sendMessage(player, C.CANNOT_AFFORD_PLOT, "" + cost);

Core/src/main/java/com/intellectualcrafters/plot/commands/Delete.java

@@ -1,6 +1,8 @@
 package com.intellectualcrafters.plot.commands;
 
 import com.intellectualcrafters.plot.config.C;
+import com.intellectualcrafters.plot.config.Settings;
+import com.intellectualcrafters.plot.object.Expression;
 import com.intellectualcrafters.plot.object.Location;
 import com.intellectualcrafters.plot.object.Plot;
 import com.intellectualcrafters.plot.object.PlotArea;
@@ -39,6 +41,7 @@ public class Delete extends SubCommand {
         }
         final PlotArea plotArea = plot.getArea();
         final java.util.Set<Plot> plots = plot.getConnectedPlots();
+        final int currentPlots = Settings.Limit.GLOBAL ? player.getPlotCount() : player.getPlotCount(loc.getWorld());
         Runnable run = new Runnable() {
             @Override
             public void run() {
@@ -52,7 +55,8 @@ public class Delete extends SubCommand {
                     public void run() {
                         plot.removeRunning();
                         if ((EconHandler.manager != null) && plotArea.USE_ECONOMY) {
-                            double value = plotArea.PRICES.get("sell") * plots.size();
+                            Expression<Double> valueExr = plotArea.PRICES.get("sell");
+                            double value = plots.size() * valueExr.evalute((double) currentPlots);
                             if (value > 0d) {
                                 EconHandler.manager.depositMoney(player, value);
                                 sendMessage(player, C.ADDED_BALANCE, String.valueOf(value));

Core/src/main/java/com/intellectualcrafters/plot/commands/MainCommand.java

@@ -139,7 +139,7 @@ public class MainCommand extends Command {
                                 if (EconHandler.manager != null) {
                                     PlotArea area = player.getApplicablePlotArea();
                                     if (area != null) {
-                                        Double price = area.PRICES.get(cmd.getFullId());
+                                        Double price = area.PRICES.get(cmd.getFullId()).evalute(0d);
                                         if (price != null && EconHandler.manager.getMoney(player) < price) {
                                             if (failure != null) {
                                                 failure.run();
@@ -158,7 +158,7 @@ public class MainCommand extends Command {
                     if (EconHandler.manager != null) {
                         PlotArea area = player.getApplicablePlotArea();
                         if (area != null) {
-                            Double price = area.PRICES.get(cmd.getFullId());
+                            Double price = area.PRICES.get(cmd.getFullId()).evalute(0d);
                             if (price != null && EconHandler.manager.getMoney(player) < price) {
                                 if (failure != null) {
                                     failure.run();

Core/src/main/java/com/intellectualcrafters/plot/commands/Merge.java

@@ -2,6 +2,7 @@ package com.intellectualcrafters.plot.commands;
 
 import com.intellectualcrafters.plot.config.C;
 import com.intellectualcrafters.plot.config.Settings;
+import com.intellectualcrafters.plot.object.Expression;
 import com.intellectualcrafters.plot.object.Location;
 import com.intellectualcrafters.plot.object.Plot;
 import com.intellectualcrafters.plot.object.PlotArea;
@@ -71,12 +72,13 @@ public class Merge extends SubCommand {
             }
         }
         final PlotArea plotArea = plot.getArea();
-        final double price = plotArea.PRICES.containsKey("merge") ? plotArea.PRICES.get("merge") : 0;
+        Expression<Double> priceExr = plotArea.PRICES.containsKey("merge") ? plotArea.PRICES.get("merge") : null;
+        final int size = plot.getConnectedPlots().size();
+        double price = priceExr == null ? 0d : priceExr.evalute((double) size);
         if (EconHandler.manager != null && plotArea.USE_ECONOMY && price > 0d && EconHandler.manager.getMoney(player) < price) {
             sendMessage(player, C.CANNOT_AFFORD_MERGE, String.valueOf(price));
             return false;
         }
-        final int size = plot.getConnectedPlots().size();
         final int maxSize = Permissions.hasPermissionRange(player, "plots.merge", Settings.Limit.MAX_PLOTS);
         if (size - 1 > maxSize) {
             MainUtil.sendMessage(player, C.NO_PERMISSION, "plots.merge." + (size + 1));

Core/src/main/java/com/intellectualcrafters/plot/object/Expression.java

@@ -0,0 +1,52 @@
+package com.intellectualcrafters.plot.object;
+
+import com.intellectualcrafters.plot.PS;
+import com.intellectualcrafters.plot.commands.DebugExec;
+import com.intellectualcrafters.plot.commands.MainCommand;
+import javax.script.ScriptException;
+
+public abstract class Expression<T> {
+    public abstract T evalute(T arg);
+
+    public static <U> Expression<U> constant(final U value) {
+        return new Expression<U>() {
+            @Override
+            public U evalute(U arg) {
+                return value;
+            }
+        };
+    }
+
+    public static Expression<Double> linearDouble(final Double value) {
+        return new Expression<Double>() {
+            @Override
+            public Double evalute(Double arg) {
+                return (arg.doubleValue() * value.doubleValue());
+            }
+        };
+    }
+
+    public static Expression<Double> doubleExpression(final String expression) {
+        try {
+            return constant(Double.parseDouble(expression));
+        } catch (Exception ignore) {}
+        if (expression.endsWith("*{arg}")) {
+            try {
+                return linearDouble(Double.parseDouble(expression.substring(0, expression.length() - 8)));
+            } catch (Exception ignore) {}
+        }
+        return new Expression<Double>() {
+            @Override
+            public Double evalute(Double arg) {
+                DebugExec exec = (DebugExec) MainCommand.getInstance().getCommand(DebugExec.class);
+                try {
+                    return (Double) exec.getEngine().eval(expression.replace("{arg}", "" + arg));
+                } catch (ScriptException e) {
+                    PS.debug("Invalid Expression: " + expression);
+                    e.printStackTrace();
+                }
+                return 0d;
+            }
+        };
+    }
+}

Core/src/main/java/com/intellectualcrafters/plot/object/PlotArea.java

@@ -56,7 +56,7 @@ public abstract class PlotArea {
     public List<String> SCHEMATICS = null;
     public Map<Flag<?>, Object> DEFAULT_FLAGS;
     public boolean USE_ECONOMY = false;
-    public Map<String, Double> PRICES = new HashMap<>();
+    public Map<String, Expression<Double>> PRICES = new HashMap<>();
     public boolean SPAWN_EGGS = false;
     public boolean SPAWN_CUSTOM = true;
     public boolean SPAWN_BREEDING = false;
@@ -225,7 +225,7 @@ public abstract class PlotArea {
         if (this.USE_ECONOMY) {
             this.PRICES = new HashMap<>();
             for (String key : priceSection.getKeys(false)) {
-                this.PRICES.put(key, priceSection.getDouble(key));
+                this.PRICES.put(key, Expression.doubleExpression(priceSection.getString(key)));
             }
         }
         this.PLOT_CHAT = config.getBoolean("chat.enabled");