I happen to know the seller, J & T Investments, LLC, and your math is a bit off on the cost per sf, just a weeeeeeeeeeeeeeeeeeeeeeeeeeee bit. I like the house, though. I could live in the carriage house.
I know they didn't actually build it for that price. ;-)
Just to break even building a 2,600 sqr ft house on a $770,000 lot and selling if for $855,000, you'd have to have a construction cost of $31.70 - about a 1/10th of normal costs, so someone lost some $.