Unlike the other games going for ridiculous prices, a mint boxed SMB is genuinely rare since nobody really had a need to purchase it and later retail variants came combined with Duck Hunt.
It's still an insanely common title.
I remember when Nintendo World Championship carts were selling for $40,000+ a few years ago (before these WATA scammers got involved) and that was by far the most rare and valuable cart... Super Mario Bros is a game that sold 30+ Million copies in various NES cart forms. This reeks of fishy dishonest speculation.