Just picked up The Legend of Zelda: Four Swords for $13.99 and Yoshi's Island for $9.99, earlier today. I'm probably going to start making game cases soon out of the DS w/ advance slot cases. This brings my total GBA collection up to 12:
FFIV
FFV
FFVI
Four Swords
Metroid Zero Mission
Mario 2
Mario World
Yoshi's Island
Harvest Moon FoMT
Mario Golf
Castlevania CoM
Advance Wars 2
There are obviously quite a few core games I'm missing. I've been keeping an eye out for the Zero collection on DS, so will get that there. There are still a ton of games I need like Mario 3, Minish Cap, Mario v. Donkey Kong, Metriod Fusion, and so many more.
I have a pretty good start though and am just snagging cheap games as I see them. I'm not the wealthiest individual, although I have a decent amount of free cash, but am also actively building my collection on other platforms.
I play everything on a silver GBA SP.