Just wanted to post that around here, Play N Trade is probably the best priced "chain" for any classic games. For my example, I'll use a loose used Pokemon red/blue cart for the GameBoy (since I've been looking for one for my son for a couple of weeks):
Play N Trade: $4.99
Game Stop: $12.99 each (when you can find one)
Movie Trading Company: $9.99
CGX: $19.99
FX Game Exchange: $21.99
Of course, if you get lucky enough to find one at Half Price Books, the price ranges from $5 to $10, depending on the store.