Steel I think is between $250-300 a ton. So if it was 400 tons 100,000 wouldn't be far off. As to where to sell it there are tons of places but in such quantity you would have some trouble unless you knew people but if your steeling a bridge you obviously thought of where to get rid of it before hand. That much steel is a lot of work and a lot of acetylene and oxygen not to mention diesel for the trucks and other equipment. On a side note depending on when the bridge was made some had a lot of wrought iron parts that is highly desirable in blacksmithing circles due to the ease of working it.