Used (Good Condition) iPhone 4 (starts $135) and 4s (starts $200) on Amazon Warehouse
Amazon Warehouse is selling iPhone 4 and 4s at a pretty good price. The last FP deal on this was over a year ago with Amazon selling the iPhone 4 at around $200, and Amazon seems to be repeating it now with the iPhone 4s at around $200

I've included the prices for the "Used-Good" condition below with links, but many of the items will have "Very good" and "like new" condition for a few bucks more on the same page.

iPhone 4
16GB Used-Good at $136
32GB Used - Good at $145

16GB used-good $135

iPhone 4S:
16GB White, $195 link
16GB Black, $200 link
32GB, Black, $195 link
32GB, White, $220 link