Amazon has free shipping on select bikes. Cheap beater bikes:

24" Ladies 18 Speed Grand Ledge Mountain Bike $44.98 shipped
26" Murray Men's Canyon Run $75 shipped
26" Boys Mountain Edge Bike $60 shipped
24" Murray Venom Boys Mountain Bike $70 shipped
26" Murray Grand Mesa Mens Mountain Bike $70 shipped
26" Murray Splash Womens Mountain Bike $70 shipped

Use coupon code 499963 for an additional $10 off on the Murray bikes.