you can buy one bus ticket at east of toronto (such as: morningside), change to subway and exit at kippling, then take another bus to the international airport, one ticket (? $2.75), cannot be cheaper. I did it 10 years ago.
downtown toronto is very crowded (i think all the downdowns are the same), if i was the mayer, I would build subways under every big street and extend them to all the surrounding cities. who cares how many people is going to take it. So I cannot be a mayer,right :))