Temporary supports can be loaned to the company for low costs, which would hardly affect the budget. This would also discourage players from being creative and trying new ideas, as it would cost money should their idea not work.
How about a variation where builders have to build from the ground up? In real life, you cannot create your bridge deck and leave it hanging there while you build your support structure around it! You would literally have to build your bridge like they do in real life. The only programming change would be to implement the excellent physics engine in teh editing mode. . .