I don't have real-time access to GameStop's current inventory or pricing. The GBA SP is a retro console (released 2003), so availability and price fluctuate wildly based on:
Expect $80–150+ USD for a typical loose AGS-101 (backlit) in decent condition. Front-lit models run $50–90. Boxed/complete sets can exceed $200. Prices have trended up steadily for years. GameStop's retro pricing is often above eBay/FB Marketplace but includes a short warranty.