I was gonna compilment you on that awesome find, but after reading that I just hope the cart doesn't work, stops working or the battery dies.
urzishra14, you and your girl are cool in my book.
Gamestop stopped accepting DC (along with NES, SNES, and Genesis) games as trade ins last year and some stores shifted their stock to bigger stores. Once everything they have left is sold, it's gone for good.
I just checked the Digital Press rarity guide and it has a rarity rating of 4 which means it's a little uncommon but far from rare.
http://www.digitpress.com/DP/cmf/game.cmf?gameid=6941
Also they have it listed at $8 for a complete copy. However I wouldn't personally pay that much for a loose...