diff --git a/src/sim/economy.ts b/src/sim/economy.ts
new file mode 100644
index 0000000..7090f5d
--- /dev/null
+++ b/src/sim/economy.ts
@@ -0,0 +1,40 @@
+import { Catalog, Game, getGame } from './catalog';
+import { Condition, ItemRef, itemKey } from './inventory';
+import { francsToCents } from './money';
+
+export const WHOLESALE_RATE = 0.65;
+export const USED_FACTOR: Record<Exclude<Condition, 'new'>, number> = {
+  complete: 0.6,
+  no_manual: 0.5,
+  loose: 0.4,
+};
+export const CASH_BUYBACK_RATE = 0.35;
+export const TRADE_BUYBACK_RATE = 0.5;
+export const MONTHLY_RENT_CENTS = francsToCents(6000);
+export const STARTING_CASH_CENTS = francsToCents(30000);
+
+export type BuyBackMode = 'cash' | 'trade';
+
+export function wholesaleCost(game: Game): number {
+  return Math.round(game.launchPriceCents * WHOLESALE_RATE);
+}
+
+export function referencePrice(game: Game, condition: Condition): number {
+  if (condition === 'new') return game.launchPriceCents;
+  return Math.round(game.launchPriceCents * USED_FACTOR[condition]);
+}
+
+/** 0.5 at the reference price, 1 at half price or less, 0 at 150% or more. */
+export function purchaseProbability(priceCents: number, referenceCents: number): number {
+  const ratio = priceCents / referenceCents;
+  return Math.min(1, Math.max(0, 1.5 - ratio));
+}
+
+export function customerMinimumAsk(game: Game, condition: Condition, mode: BuyBackMode): number {
+  const rate = mode === 'cash' ? CASH_BUYBACK_RATE : TRADE_BUYBACK_RATE;
+  return Math.round(referencePrice(game, condition) * rate);
+}
+
+export function priceOf(prices: Record<string, number>, catalog: Catalog, item: ItemRef): number {
+  return prices[itemKey(item)] ?? referencePrice(getGame(catalog, item.gameId), item.condition);
+}
diff --git a/tests/sim/economy.test.ts b/tests/sim/economy.test.ts
new file mode 100644
index 0000000..a9e5048
--- /dev/null
+++ b/tests/sim/economy.test.ts
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+import { describe, it, expect } from 'vitest';
+import {
+  customerMinimumAsk, MONTHLY_RENT_CENTS, priceOf, purchaseProbability,
+  referencePrice, wholesaleCost,
+} from '../../src/sim/economy';
+import { getGame } from '../../src/sim/catalog';
+import { testCatalog } from './helpers';
+
+const catalog = testCatalog();
+const sml = getGame(catalog, 'sml'); // 3796 cents
+
+describe('economy', () => {
+  it('buys from the wholesaler at 65% of launch price', () => {
+    expect(wholesaleCost(sml)).toBe(2467);
+  });
+
+  it('derives used reference prices from the condition', () => {
+    expect(referencePrice(sml, 'new')).toBe(3796);
+    expect(referencePrice(sml, 'complete')).toBe(2278);
+    expect(referencePrice(sml, 'no_manual')).toBe(1898);
+    expect(referencePrice(sml, 'loose')).toBe(1518);
+  });
+
+  it('maps price ratio to purchase probability', () => {
+    expect(purchaseProbability(1000, 1000)).toBeCloseTo(0.5);
+    expect(purchaseProbability(500, 1000)).toBe(1);
+    expect(purchaseProbability(1500, 1000)).toBe(0);
+    expect(purchaseProbability(3000, 1000)).toBe(0);
+  });
+
+  it('computes what a customer accepts for a buy-back', () => {
+    expect(customerMinimumAsk(sml, 'complete', 'cash')).toBe(797);
+    expect(customerMinimumAsk(sml, 'complete', 'trade')).toBe(1139);
+  });
+
+  it('uses the player price, else the reference price', () => {
+    const item = { gameId: 'sml', condition: 'new' as const };
+    expect(priceOf({}, catalog, item)).toBe(3796);
+    expect(priceOf({ 'sml|new': 3500 }, catalog, item)).toBe(3500);
+  });
+
+  it('charges 6000 F of rent per month', () => {
+    expect(MONTHLY_RENT_CENTS).toBe(91469);
+  });
+});