(source aaronsc at Flickr.com)
This is the Saputo Stadium in Montreal built for $14.1 million dollars. If this was build to CFL field dimensions instead of soccer then the capacity would be about 33% larger (about 17,500 instead of 13,034 at Saputo). This is based on the perimeter of a CFL field being 430 yards (150 + 150 + 65 + 65) versus 320 yards for soccer (100 + 100 + 60 + 60). If the individual chairs were replace with bench style seating then that would increase capacity by about 5% - 10% so it would be about 19,000. I would estimate the price to be $20 million - $25 million. But I think that the roof should be over all of the seats instead of just one section. Or at least make it over the two sideline seating sections. However I think that the capacity would have to be increased to about 30,000. I don't know how much that would add to the cost. How about a double tier of seats along the sidelines with the upper tier cantilevered to provide a roof for the lower level and then have a roof for the upper level (this might greatly increase the cost since the design would become more complex). Maybe the end zones could be left open for temporary seats in the event of a Grey Cup being played. So have 30,000 seats on the 2 sidelines.
What interests me about this stadium is that it was built at a low price but still has a roof over a section of stands.