Just wait. I am convinced that the SP will drop another $20 soon, since the GB Micro is being released. And you could buy a used one for $50, or wait and maybe buy a brand new one for $60.
However, Best Buy is also selling a GBA SP with Final Fantasy Tactics and Metroid: Zero Mission all for $99. Those are two excellent games which still both go for $25-$35 usually, and a system that currently goes for $80.
Hybrid Mode
