If you might ever be interested in playing n64 games on-console, there is always the option of a n64 flashcart with a gb transfer pack. Yeah, that'll probably be way more expensive than $50, but if you were to be using it a lot for playing games besides just doing save data backups, I'd say that may make up for the cost.
Otherwise if you don't necessarily care about keeping the cartridge, there may be someone you could mail the game to who could backup the save data for you. Then depending on how much you care about the cart itself, you could have them send it back to you afterwards, or let them keep it in exchange for their service.