At this point, if I had this project in my lap, I'd be more concerned about wether or not the whole thing would work. K-F engineers used the Club Coupe/Business Coupe body for the Kaiser Californian K5122 convertible development because the coupe had more rear deck space than the sedan (check out trunk hinges relative to the rear window location...you will find more rear deck space with the Coupe) and that gave better access to space for the back seat and the top mechanism. If you haven't done so already, I would suggest you put the rear seat back in this car to see how much space you have for the top and the top mechanism just to make sure it will clear properly when the thing goes up and down. You may find the seat back may not fit right if my guess is right as to how much space the top will need.
As for cost, it's virtually impossible to figure out what the car might be worth as finished. First, you don't know at this point how much money you will put into it and what the finished car will be like. Second, as this is not a factory prototype or other factory special build car, it would be treated much the same as a customized production car.
While I am not a factory stylist or body DRE (Design Responsible Engineer as they care called at GM) I have been around this business long enough to have some insight.