Everyone always complains how each season's accom is "worse than ever", but having done 6 winters there I can tell you there will always be something! Sure, it might take some work, but you'll find something eventually. If you're arriving in Jan you can hopefully score a bed/room from people who have left early. I'd budget for at least $600-700 per month for a shared room, and anywhere from there up to $900ish for your own room. I'd recommend maybe booking some days somewhere (e.g. hostel/lodge) when you first arrive to play it safe. The FB page "Whistler housing rentals for locals" is good (although you have to act fucking fast on the ads), Craigslist can also have some spots (but beware of scams). Whatever you do don't pay money to anyone before you get there and see the place. Classic stitch up...
https://www.youtube.com/watch?v=kMFjVI_GbRg