Not too sure, as most of the GBA games I see in the wild are shitty loose licensed games. I do know Ninja Five-0 is extremely rare, if you have that cib it's worth like $200 (I'm so glad I grabbed one!). And I'm sure the Nintendo games cib have held their value fairly well, Zelda LTTP cib is going for $80 on amazon for instance. And the Castlevanias are definitely getting up there.