GameStop doesn't publish fixed prices for accessories like the Xbox 360 rechargeable battery pack. The offer you'll get depends entirely on condition, current demand, and the individual store's inventory needs.
A brand-new, sealed pack in original packaging? You'll get the top tier. A loose, scratched pack with a frayed cable might only net a few dollars—or be declined entirely. If you have the official play-and-charge kit (battery + cable), keep them together; incomplete sets are worth significantly less.
Pro tip: Clean the contacts with a dry cloth before trading it in. A little presentation effort can bump the offer up a tier.