regulation is 150lbs of concrete, i never use that much, but my hsp says its 'required' and yea, 400$ is not a bad price for the work he quoted you, and his day off, yea... its gonna be a long install, thats reasonable too, most of us work 6day work weeks, so to make that a 7, i think i would want 400$ too lol.
I agree, with what you say..I've gotten away with 2 bags or 100lbs. (quickcrete red bags)