Prices vary by model, condition, box/papers, and region. Recent eBay sold averages (USD, loose):
Boxed/CIB: 2–5× loose price. Limited editions (Pokémon, Zelda, 24k Gold) command premiums.
Buy safely: eBay (top-rated), r/GameboyMarketplace, r/GameSale, local retro shops. Test before buying if possible.