What you are looking for doesn't exist. The cost of manufacturing something with any remotely decent silicon, for a niche device that doesn't have the benefit of economies of scale, is significantly higher than it used to be.
There's a reason all of the AliExpress "UMPC" devices being posted are over $1000. "$300 is pretty steep" is a ridiculous statement for a niche device like this given the modern landscape of manufacturing costs.
$300 is CHEAP. It is literally DIRT CHEAP. You will not find any modern, currently manufactured device for cheaper because it does not exist, and it cannot exist.