> But 8x10 for $20,000 just goes to show how expensive USA is for construction.

Labor is expensive. You could probably close to double the size for an additional $10,000, you’d need another yard and a half of concrete for the foundation (6” depth*80 sqft = 40 cubic feet) and more material for the building, but not twice as much material, or labor. Electrical service could stay the same.l and the mini-split wouldn’t have to be much bigger.