The prices for the Game&Watch games change all the time. This depends on the demand and the number of games on the market, the rarity and the condition. A very rare game is the Game&Watch Panorama Screen Mickey Mouse. It's very difficult to find this game including its box. Regarding the auction database, there were four games only sold this year so far.

That could explain the rather high end price of this Mickey Mouse here. The game was sold on October 31st and it went for USD $1'235.00! The average price for such a game usually is about USD $600.00 to $700.00. It looks like some collectors just have been waiting for this a game to show up on eBay...

==> Link to the auction or Download as PDF