diff --git a/backend/src/lib/scraps.ts b/backend/src/lib/scraps.ts
index babcaac..e825f32 100644
--- a/backend/src/lib/scraps.ts
+++ b/backend/src/lib/scraps.ts
@@ -91,23 +91,22 @@ export function calculateShopItemPricing(
   };
 }
 
-const MIN_ROLL_COST_PERCENT = 0.15;
-
 export function calculateRollCost(
   basePrice: number,
   effectiveProbability: number,
   rollCostOverride?: number | null,
+  baseProbability?: number,
 ): number {
-  // If admin set a manual roll cost, use it
   if (rollCostOverride != null && rollCostOverride > 0) {
     return rollCostOverride;
   }
-  // Roll cost scales with effective probability (including upgrades).
-  // Floor at 15% of item price prevents exploitation on rare items where
-  // base probability is very low (e.g., 2% base → flat cost of 6 scraps).
-  const scaledCost = Math.round(basePrice * (effectiveProbability / 100));
-  const floorCost = Math.round(basePrice * MIN_ROLL_COST_PERCENT);
-  return Math.max(1, scaledCost, floorCost);
+  // Roll cost is fixed based on base probability, does not scale with upgrades.
+  // Rare items (baseProbability < 15%) get a 20% floor.
+  const baseProb = baseProbability ?? effectiveProbability;
+  if (baseProb < 15) {
+    return Math.max(1, Math.round(basePrice * 0.20));
+  }
+  return Math.max(1, Math.round(basePrice * (baseProb / 100)));
 }
 
 export function computeRollThreshold(probability: number): number {
diff --git a/backend/src/lib/shop-pricing.ts b/backend/src/lib/shop-pricing.ts
index 6b3b81a..88c53f0 100644
--- a/backend/src/lib/shop-pricing.ts
+++ b/backend/src/lib/shop-pricing.ts
@@ -64,8 +64,7 @@ export function computeItemPricing(
     );
   }
 
-  // Roll cost at base probability (includes 15% floor to prevent exploitation on rare items)
-  const rollCost = calculateRollCost(price, prob);
+  const rollCost = calculateRollCost(price, prob, undefined, prob);
 
   const threshold = computeRollThreshold(prob);
   const expectedRollsAtBase = threshold > 0 ? Math.round((100 / threshold) * 10) / 10 : Infinity;
diff --git a/backend/src/routes/shop.ts b/backend/src/routes/shop.ts
index 69847c1..c84aee8 100644
--- a/backend/src/routes/shop.ts
+++ b/backend/src/routes/shop.ts
@@ -574,11 +574,11 @@ shop.post("/items/:id/try-luck", async ({ params, headers }) => {
         100,
       );
 
-      // Roll cost scales with effective probability (including upgrades)
       const rollCost = calculateRollCost(
         currentItem.price,
         effectiveProbability,
         currentItem.rollCostOverride,
+        currentItem.baseProbability,
       );
 
       // Check if user can afford the roll cost
diff --git a/frontend/src/routes/admin/shop/+page.svelte b/frontend/src/routes/admin/shop/+page.svelte
index bf15129..f74bd5b 100644
--- a/frontend/src/routes/admin/shop/+page.svelte
+++ b/frontend/src/routes/admin/shop/+page.svelte
@@ -90,16 +90,20 @@
 	const SCRAPS_PER_DOLLAR = SCRAPS_PER_HOUR / DOLLARS_PER_HOUR;
 
 	// Must match backend calculateRollCost exactly
-	// Roll cost scales with effective probability (including upgrades)
 	function calculateRollCost(
 		basePrice: number,
 		effectiveProbability: number,
-		rollCostOverride?: number | null
+		rollCostOverride?: number | null,
+		baseProbability?: number
 	): number {
 		if (rollCostOverride != null && rollCostOverride > 0) {
 			return rollCostOverride;
 		}
-		return Math.max(1, Math.round(basePrice * (effectiveProbability / 100)));
+		const baseProb = baseProbability ?? effectiveProbability;
+		if (baseProb < 15) {
+			return Math.max(1, Math.round(basePrice * 0.20));
+		}
+		return Math.max(1, Math.round(basePrice * (baseProb / 100)));
 	}
 
 	// Must match backend computeRollThreshold exactly
@@ -165,8 +169,7 @@
 			const boostPercent = k * boostAmount;
 			const effectiveProbability = Math.min(baseProbability + boostPercent, 100);
 
-			// Roll cost scales with effective probability (including upgrades)
-			const rollCost = calculateRollCost(price, effectiveProbability, rollCostOverride);
+			const rollCost = calculateRollCost(price, effectiveProbability, rollCostOverride, baseProbability);
 
 			// Cumulative upgrade cost (geometric series)
 			let upgradeCostCumulative = 0;
diff --git a/frontend/src/routes/shop/+page.svelte b/frontend/src/routes/shop/+page.svelte
index f0b4d5b..be06c4f 100644
--- a/frontend/src/routes/shop/+page.svelte
+++ b/frontend/src/routes/shop/+page.svelte
@@ -26,8 +26,10 @@
 		if (item.rollCostOverride != null && item.rollCostOverride > 0) {
 			return item.rollCostOverride;
 		}
-		// Roll cost scales with effective probability (including upgrades)
-		return Math.max(1, Math.round(item.price * (item.effectiveProbability / 100)));
+		if (item.baseProbability < 15) {
+			return Math.max(1, Math.round(item.price * 0.20));
+		}
+		return Math.max(1, Math.round(item.price * (item.baseProbability / 100)));
 	}
 
 	let selectedCategories = $state<Set<string>>(new Set());