Scott,
I would like to encourage you to build on your own. There are a lot of resources on the net to pull from. A 14'x42' is perfect for an additional dwelling. You are correct. The prices contractors are charging now is outrageous. My older brother and I built his home where he has now retired, in 1987. It was a two car garage 'kit' that he had them expand 8 feet on the back to accommodate a second bathroom. I think he was all at less than $20k, well plumbing, electrical house and a/c. He has never had a mortgage. It was a first time for both of us; but we got it done.
You don't have to do it all yourself, electrical and plumbing can be contracted; or anything else you feel is not your skill set. However, long rectangles are really easy to frame. The foundation is already poured. You are further along than one might think.
If you are willing to explore that possibility more, there is a lot of knowledge on here willing to help and support. Is there anything in particular that would keep you from doing a simple one level home?