Generally, too much. (you are talking about metered cabs, right?)
On the other hand, there are companies that do airports only. Airways Transat is about $79 before tip.
There are other options like the GO Bus. I hear you can take the 2 GO buses to get to the airport, but they only run on weekdays.
I usually take the GO Bus to union (about $8), and then take a shuttle from the Royal York (
http://www.torontoairportexp ress.com/ $21.95). The shuttle has on-board WiFi, which is nice. Total of $30 one way; it's not too bad, plus the trip's pretty comfortable.
The absolute cheapest but worst way to get to the airport is: 1) GO Bus to Union ($8); 2) TTC from Union to St George ($3); 3) TTC from St George to Kipling. 4) TTC shuttle from Kipling to airport (192). It'll only cost $11 but the travel time is 2.5 hours.