If you're looking for a low price, don't overlook Nashbar and Performance Bike. They always have constantly rotating sales and coupons, and if you strike at te right time, you can get an incredible bargain. I think I have bought 5 or 6 rear racks total from these two combined, always for less than $25, and I have no complaint about their design or serviceability. We're talking generic racks here, not mounting system specific. That's always been good enough for me.
Nashbar even foreshadows their coupons, so you can bide your time and strike when the most advantageous coupon is in effect.