If you're looking for a range toy, you should look at NAA mini revolvers (about $175) and those flare gun-lookin' Cobray derringers (like $80). Neither is a weapon I'd want to go to war with, but they're fun and cheap and the NAA is a head turner, even though its the opposite end of the spectrum.
Now, if you're looking for something for serious self defense, i share your opinion. Personally, I'd rather have a small revolver or auto. A 2-shot derringer that has the same frame size and weight as a small auto is kind of pointless to me. I mean, even if the auto fails, you're only getting one less shot lol. And the prices are about the same.
If you're set on a traditional derringer, Id shop around pawn shops and gun shows and see if you can find a good deal. I bet you could find one under priced in a pawn shop, once you haggled the guy down.