GameStop.com discounts a selection of used games for Xbox 360, with prices starting at 99 cents. Opt for in-store pickup to dodge shipping fees, which start at $3.50. (Click "Pickup@Store" on the product page to check availability.)

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A couple best bets:
-- Used Gears of War 2 for Xbox 360 (pictured) for 99 cents (low for a used copy by $5)
-- Used Battlefield: Bad Company 2 for Xbox 360 for $1.99 (low for a used copy by $4)