Lets calculate how much you save per hour. you put 7 hours each weekday, and plus maybe work longer hours on weekends. lets assume averagely you worked 8hrs/day x 30day/mon x 7mon = 1680 hrs. and you save $13100 / 1680hrs = $7.8/hr.
The contractor can put lien on the house if he didn't get paid for the honest labour. The property owner has to pay it sooner or later. But if the property owner get screwed by the contractor, it is very difficult to get the money back.