More than you want to spend but maybe a monthly rate.
Cheapest on the beach although some of Lilly's are close.
Don't know what you will be doing for one month, but if you are going to be just hanging living the San Pedro life you might want a nice beachfront upstairs porch & fridge/stove to save $ on food & beverages. Also there is some % charged
over the cost of the rooms anywhere ( except private maybe)http://ambergriscaye.com/rubys/beach.htm
have seen many young divers stay herehttp://ambergriscaye.com/marthas/index.html
Have seen many college age stay here
Don't know if they are still available but Milos Store used to rent rooms with shared bathrooms for cheap.http://ambergriscaye.com/phonebook/9.html
For me, I need my own bathroom and would make other $ sacrifices to get one! However, my brother stayed at Milos and was fine. Also Caye Caulker might be an option for you. My friend stayed there in a rooming house with shared bath for like $10 US years ago -but I'll bet there are still great deals on Caulker if you're willing to share baths or not.