The total cost, according to Tennis Canada is around 40 million dollars (Canadian), that's a little more than 25 million US dollars.
I don't know what they are thinking by making it so big, I believe that few courts in the world have 12,500 seats.
They will have 15 outdoor courts, I believe the GSs have around 19.
they are not planning to use the indoor courts for the tournament, but for practice and stuff like that, I guess they could use them when it rains though (it's probably cheaper to build another stadium than a retractable roof, as Toronto painfully learned with the Skydome).
No idea about what they will do, if they will use it for other activities or not.