For me I found so many flakes going in to share a storage unit. It would've worked out to stupid cheap for the season, but all I found were broke motorcyclist with a dream.
I think it would have to be a friends situation, or perhaps a few reputable forum members here type scenario. I could see how just putting an ad up on Kijiji or something would yield iffy results..and I really wouldn't want anyone I don't trust to have a key to where my bike is stored, either. But at $90/month that GP bikes is quoting I think you can rent a small unit that would fit a bike all to yourself with some money left over, no? I don't know, it's been a long time since I've looked at rates.
In the end if I went the storage unit route I'd probably quietly slip it in one evening after dark when nobody is there, lock the door, and be very picky when I open it...AKA, don't advertise there's a bike in there. Not that most storage places aren't reasonably secure, but you know how it goes - out of sight, out of mind.
I'm lucky to have a nice well secured garage myself, but I know lots of people in the city aren't so equipped.