Dollar-no-object, you really don't want anything that can't run the latest Android versions. That says you want either the current Nexus 4 or the previous Galaxy Nexus. A Nexus S is the oldest that you might want, and that's starting to make compromises.

Poking around on eBay, the "buy it now" new-in-box Nexus S units seem to be going for $180, give or take. Used phones seem to be under $150, if you're persistent.

The Galaxy Nexus, used, seems to be clocking in around $250, which is pretty sad when a brand new Nexus 4 can be had for $299 directly from Google.