EDIT - Major oversight brought to my attention.
I've got just about every GBA RPG available in the US, and I have no clue if you'll be able to find some of the older ones at all, cheap or otherwise.
Phantasy Star Collection - a must own compilation of 3 very classic games, 2 of them awesome. Sadly missing the final chapter of the Phantasy Star saga, but still a great value.
Breath of Fires. 1st was garbage, 2 was halfway decent. Low end rpgs all in all.
Lunar was fun and charming, though bug ridden and stripped down.
Lufia - RoL was easily the worst in the series. Very... unprofessionally done. But sorta fun.
CIMA was fun though brain dead. Pathfinding can be a bitch, as is NPC AI. Not a true RPG.
Golden Suns. 1st was mediocre, 2nd is the system's best traditional RPG imho.
Fire Emblem & Shining Force. Dig "chessboard" style strat-rpgs? Must owns.
Sword of Mana, Shining Soul 2. Decent to good action rpgs.
Pokemon Ruby/Sapphire. Excellent fun, insane replay, huge length.
Tactics Ogre - Knight of Lodis. Like "tactics" style strat-rpgs? Best on the GBA.
FF Tactics Advance. Fun but silly. Not as good as TO - KoL imho.
Baldur's Gate Dark Alliance. Fun and rewarding Diablo-style dungeon romp.
Eye of the Beholder. Very odd blend of 3rd edition rules, classic EotB 1st person dungeon exploration, and classic SSI Goldenbox game combat style. Mediocre game.
I have to mention:
Dragon Warrior III - GBC release. One of the greatest RPGs currently on any handheld.
Dragon Warrior I&II - GB/GBC release. Almost as good as DWIII.
There are quite a few more, but I think that's a decent list of what's good and what's not. I'd be happy to expand upon any of my above comments.