I like the type of stadium below (Stanford Stadium). The capacity is much greater than required at 50,000 seats but if you imagine it without the end zones seating, fewer rows along the Grandstand side, stretched out for the CFL, and no upper tier along the other side then it would be about 20,000 seats. Then there would be lots of room for expansion in the future. This stadium, including demolition of their old stadium was built for $90 million dollars and opened 4 years ago -
http://en.wikipedia.org/wiki/Stanford_Stadium . In order to get an idea of how such a large stadium can be built so cheaply then you should check out this YouTube video -
http://www.youtube.com/watch?v=amWNxBSLGJk . I think that Halifax could get a very nice stadium for $40 - $50 million using the Stanford type of construction method. This is the type of stadium that I was thinking of when I made this 3D model -
http://www.youtube.com/watch?v=zTVTTrSHXn8&feature=fvsr . However, even my 3D model shown in the animation is much too big at 31,000 seats. (It has a roof for protection from the rainy Maritime weather.)