diff --git a/constants.js b/constants.js
index c0bbb6a..8c6e14a 100644
--- a/constants.js
+++ b/constants.js
@@ -317,7 +317,7 @@ function createProdUpgrades() {
         "desc": contents.desc,
         "icon": buildings[key].icon,
         "cost": {
-          "food": buildings[key].cost * 5 * Math.pow(10, counter - 1)
+          "food": buildings[key].cost.food * 5 * Math.pow(10, counter - 1)
         },
         "effects": [
           {
diff --git a/gorge.js b/gorge.js
index 4d95b37..1f4f991 100644
--- a/gorge.js
+++ b/gorge.js
@@ -116,27 +116,26 @@ function productivityMultiplierOf(type) {
 function productivityOf(type) {
   let baseProd = makeCost(buildings[type].prod);
 
-  baseProd.food *= productivityMultiplierOf(type);
+  baseProd = scaleCost(baseProd, productivityMultiplierOf(type));
 
-  baseProd.food *= getGlobalProdBonus();
+  baseProd = scaleCost(baseProd, productivityMultiplierOf(type));
 
-  baseProd.food *= belongings[type].count;
+  baseProd = scaleCost(baseProd, belongings[type].count);
 
   return baseProd;
 }
 
 function makeCost(source) {
   const empty = mapObject(resourceTypes, () => 0);
-  Object.preventExtensions(empty);
-  return {...empty, ...source};
+  return deepFreeze({...empty, ...source});
 }
 
 function addCost(cost1, cost2) {
-  return Object.keys(resourceTypes).reduce((o, k) => ({ ...o, [k]: cost1[k] + cost2[k]}), {});
+  return deepFreeze(Object.keys(resourceTypes).reduce((o, k) => ({ ...o, [k]: cost1[k] + cost2[k]}), {}));
 }
 
 function scaleCost(cost, scale) {
-  return Object.keys(resourceTypes).reduce((o, k) => ({ ...o, [k]: cost[k] * scale}), {});
+  return deepFreeze(Object.keys(resourceTypes).reduce((o, k) => ({ ...o, [k]: cost[k] * scale}), {}));
 }
 
 function costOfBuilding(type, count = 1) {
@@ -144,7 +143,7 @@ function costOfBuilding(type, count = 1) {
 
   while (count > 0) {
     let baseCost = makeCost(buildings[type].cost);
-    baseCost.food *= Math.pow(1.15, belongings[type].count + count - 1);
+    baseCost  = scaleCost(baseCost, Math.pow(1.15, belongings[type].count + count - 1));
     total = addCost(total, baseCost);
     count--;
   }
diff --git a/util.js b/util.js
index c9b90aa..be32c59 100644
--- a/util.js
+++ b/util.js
@@ -20,7 +20,7 @@ function round(val, places = 0) {
 }
 
 function mapObject(obj, func) {
-  return Object.keys(obj).reduce((o, k) => ({ ...o, [k]: func(obj[k])}), {});
+  return deepFreeze(Object.keys(obj).reduce((o, k) => ({ ...o, [k]: func(obj[k])}), {}));
 }
 
 function deepFreeze(object) {