The first 2 Fallout games come to mind, and I believe you can buy them for a mere $5.99 each at Good Old Games (www.gog.com). If you decide to get them though, I'd strongly suggest playing them in order. Not so much because of needing to know what happened in the first one to enjoy the second, but because while they are similar, the second game is quite a bit more polished in various areas, which makes the lack of some polish in the first one all the more obvious. Both games are great though, and I recommend them heartily.
The Jagged Alliance games comes to mind as well, although they are more strategy games with a dose of RPG in them.
Baldur's Gate 2 is sort of turn based. You can pause whenever you want to in order to issue commands, and you can also set it to auto-pause after every action in combat, making it pretty much turn based. Knights of the Old Republic is also semi-turn based by using pause functions.
Space Rangers 2, which you can get on Impulse, has RPG elements, but not sure if that qualifies since you didn't say much about what sort of RPG you are after, other than that it should be turn based.
I suppose a lot of JRPGs that you find on consoles would be considered turn based or semi-turn based, like the Final Fantasy series (X is fully turn based). Vagrant Story for Playstation 1 is a good one, Suikoden II is another good one I've played. If you meant more in the vein of Strategy RPGs, then I guess some titles to check out would be Fire Emblem, Final Fantasy Tactics, Ogre Battle, Disgea, the Front Mission series, Vandal Hearts and probably many more I'm forgetting.