It's absolutely worth it. I'm in the same situation, I actually rent a parking space for the same amount so I can offer to guests. I actually charge them exactly what I pay weekly. So far, it has paid for itself. There will be a week or so no one wants the space, in which case I have to pay it, but my space is shared between 2 apartments.
You might want to charge a bit more to offset the times when it won't be rented. I'd say you could easily charge $25 per day for downtown Toronto and that would be reasonable.
The parking is not so much for people to drive while they're in town, but rather for people who drive to Toronto and need the space for their car until they leave.
If i could actually rent another space I would, that's how many inquiries and requests I get about parking and lose bookings because the parking space is reserved by the other unit.