I have heard that $300 figure over and over for ADU custom construction. If you built a lot more there should be some economies of scale but Id use that for a single small house of say 1200 to 1600 sq ft. There are lots of fixed costs so the smaller you build the higher the cost per sq ft.
High end custom finishes are actually very expensive. Cheap imports from China have lowered the cost of stone and cabinets that have a high end look for not much more. Higher end finishes could easily add another $100K+ to the cost of something once you start dealing with custom cabinetry, high end flooring, appliances, lighting and plumbing fixtures etc. But I guess it all comes down to what one considers high end.
