Neither the Sega Game Gear nor the Sega Nomad has been sold new in decades. Prices on the secondary market vary wildly based on condition, model revision, included accessories, and regional variant. Here's a realistic picture of what you'll pay today.
For a precise estimate, specify: which system, model variant, condition (working/untested/parts), included accessories, and your budget. A recapped, tested Game Gear with a screen protector is a very different purchase than a "powers on, no video" Nomad.