I waited until I saw comments from fellow CRG members and to say the least in my opinion and the opinion of one of my friends who does restoration for a living,,,,,,,,,, you got took. He just completed a 1967 convertible (rotisserie) and the total cost including rebuilding the 327 with some extra pop was $30K!!!!!!! This cost included front fenders, full rear quarters,roof, floor in trunk and a complete interior and all electrical wiring. Like you stated, your car is nothing special in the sense of it being a Z, Copo or a Yenko so I can not fathom how this shop can "justify" these costs.
Tell us that you are in error and the cost is no where near that amount.