The price of his battery and power converter are pretty insane, and a Raspberry Pi is convenient but overkill. Given a more price-conscious part choice and at least some volume (since what's the point in building only one cartridge), you should be able to get that to $5 plus display fairly easily. Suitable displays can be found for $10 from Amazon (shipped from China).
So you can probably home-build it for $15. If you're a startup in Shenzhen it might be feasible to sell them assembled for $10 and make a nice profit if you can sell a million of them.