Best Electronics...who I've bought from before...and are pretty good in spite of their sometimes high prices are very good...have that demo cart at 20 bucks...so I'd assume its worth a little bit less. They're surprisingly common.
True story: That demo cart stopped me from buying a Lynx back when it first came out. If i recall, its an unplayable demo...long story short, the game gear demo in Toys r Us (G Loc, I think) was. That sold me on it.