Oh man, 'Project Dream' is like the holy grail of lost media in gaming. Last I heard, there was a guy who claimed his uncle worked at Rare and had some concept art but no prototype. Would give my left joystick to see anything from that era tbh.
5 bucks? Man, that's a steal. Make sure to dump the ROM carefully! These old betas are super fragile, lots of them have bit rot and might not survive multiple playthroughs. And when you're taking screenshots, document everything. Hell, if you find the right buyer, that cart might net you …