For the complete game as a rom, I would be willing to pay around $10.00 US. Maybe even $15.00 if it has an amazing FM soundtrack. In terms of purchasing rom files, I believe that your customer would have less of a "collector's mindset" than a gamer who purchases a physical cart with box and manual. I am personally a ROM customer, and I like playing on real hardware with my games on an everdrive. So, I look for presentation, features like high score saving, and games that feel like they could have been a commercial product back in the nineties. Some of the best ROMs that I have purchased here on itch have been small-scale arcade games with a high level of polish. I hope that helps, as I really enjoyed the demo.