The construction prices you quoted are not correct at least in the Bay Area. And I am guessing $150/sq ft achievable at scale.
Pricing ADUs is tricky because the some of things in a house cost same for a small to medium size home. For example, the costs of plumbing, electrical, kitchen, bathroom, etc are more of less the same between a 500 sq.ft. house and a 1000 sq. ft house.