Supply and demand...
Want to know why the Droid is so cheap on here? Its because Verizon was literally giving them away and completely saturated the market. The sheer number of them out there has made them relatively worthless. There were too many out there for them to be a valuable commodity anymore, and thus, their value plummeted. The amount they are giving you for the replacement is simply enough to cover some of the cost of the parts they would have to manufacture otherwise for the Droid 2.
As for the Droid Eris, I can understand why it is pricey. If you have one that works and can legitimately be used as a replacement, you have a rare phone. Verizon wants that for people who need a replacement. Therefore, you have a valuable phone.
And for the iPhone. Interesting that Verizon would even take them in, but then again, they can turn around and sell those back to Apple/AT&T for a markup on the amount they pay you, and they still make some money off it. They lowball you in order to make a profit.