In November 2005, Christ Church in New York sold their air rights for a record US$430 per square foot. Let's call that CAN$5000 per square meter. At $250,000,000 per km, you need to sell 50,000 sq.m. per km of air rights (50m wide corridor). You might be able to pay for part of a station by selling it's air rights, but you won't fund a subway on it.
Better than nothing? That depends if you own it already or not. As the TTC doesn't own many titles along proposed corridors, you're looking at government buying with one hand and selling with another. Why not just skip building the subway altogether and buy every undeveloped piece of land that comes on the market in Toronto and sell the air rights?
Listen to what is said, not what you want to hear.