In Quebec, the estimate starts at 500m. When it's time to build, the estimate gets raised 50-85%, and then when it's complete, you're looking at min a 1b+ stadium. Look at the the turcot interchange, and every other piece of infrastructure that's been built lately. If the US dollar gets to 1.20+ , I'm not sure how viable any MLB franchise will be here.
But realistically figure 50-75m to break the lease, 750-900m to build a stadium with land, and what if Rays ownership just decides to move, not sell? Then there's no cost of a team, it's just relocating an existing franchise. If they are selling, then it's 500-600m for the franchise. So, in all you are looking at min 1.5b to get a Montreal Expos team back in Montreal if the Rays are selling out and plan to move here.