Hey! Welcome to PC :)
https://www.ebay.com.au/itm/Original-Gameboy-Pokemon-Red-Comes-With-Mew/263516057302?hash=item3d5ac7eed6:g:Gw8AAOSwb3laGn6~
This one mentions Mew! Buuut says only $80... and isn't sold, so I don't think it would be high in general but to a collector I'd say it'd be worth a bit. People on ebay aren't specifically looking for an original Red with a Mew unfortunately. :( Hm. You would need a hardcore collector and I can't think of anyone on here that might be except maybe Laslow? I'll tag him to this thread anyway and see if he has any ideas. Keep in mind the selling can't actually happen on this forum itself but he can probably suggest places that you might be able to sell it to!