cheapest is probably gropius dorms. then hastings dorms. these two also best in terms of location/proximity to law school though the rooms are small and... they're dorms.
north is another dorm, further from the law school, but bigger rooms and you get your own bathroom.
harvard-affiliated apartments have a couple options for law school students, though i've heard it's sometimes hard to get them...
and then craigslist is always good for off-campus apartments. where you can have your own kitchen, bathroom, and pet.