If you aren't here before december 1st, expect to live in employee housing. It gets very difficult to find places after november and most of the rooms left are in the $700 a month + range.
check the mammoth times classifieds as well as the mammoth mountain forum for room/housing listings.
Expect to pay around $550 - $700 for rent after utilities.
Avoid "the slopes" as it's very family oriented and requires you to drive to everywhere. Also avoid if it says "old mammoth - passed the firehouse."
Cheaper housing tends to be in what is called "the ghetto" it includes Manzanita, Mono, Jauquine, and Lupin. Anything "near canyon" tends to be pretty nice. "near eagle" is nice as well, but you'll do a lot more walking as the shuttle hours are weird. Near Vons is cool too, but if over 21, prepare to leave the bar early enough to catch the trolley, or walk a mile and a half home.
Employee housing is actually pretty nice and tends to clear out quite a bit by january. I know a lot of people in double rooms that end up without a roommate. And if you're under 21... it's wayyyyyy easier to make friends.