I think the HP Stream 7 is one of the cheapest $80 Windows tablet from a known brand like HP, and Dell. There are cheaper Windows tablets from lesser known brands, but they maybe less reliable, and harder to exchange if they break before the warranty period.
There is the $50 Amazon Kindle Fire, and cheaper Android tablets which can be used for gaming. But, you only can play Android games, emulators, and can't play games from Steam, Gog, and Origin.