If you haven't picked up Yoshi's Island or Super Mario Advance (aka SMB2), both have been re-released as Player's Choice editions. So they retail brand new at $20 bucks, which is sure to knock the used editions down in price as well.
Surprisingly, I found out the Kim Possible 2 GBA game is actually good, unlike nearly all licensed titles based on kids' cartoons. I know nothing of the license, but it plays somewhat like you'd expect a good Batman game to play.
There are some gripes (collectables you can collect for who knows what reason; relatively short; enemies go down with ease), but it has great graphics, control, a good variety of gadgets and large levels. I hear the 3rd game is good too, but that the 1st game and the DS title are passes.
For an actual, DC-blessed Batman game, Rise of Sin Tzu is the best of the lot (Vengeance is a sad wannabe of the SNES Batman and Robin, right down to a Trouble in Transit knockoff), but RoST improves on it in every way. Don't confuse RoST with its console counterpart (which is a straight up beat 'em up), although they do share the same basic plot.
I haven't played the GBA Begins, though, yet its 62% on Gamerankings doesn't bode well.
Good places to check for cheap GBA games (if applicable):
Circuit City (found the KP2 and Batman RoST games there for under 12 bucks brand new)
K-Mart (stuff that doesn't sell often gets marked down after so much time)
Sears (people probably don't buy games there; I got NES Classics titles for $7.99 new, Metroid ZM for $2.99, Mario vs. DK for $14.99, etc).