Rent for accommodation is determined based on a points system, with points given for location (college being the best), size, and features (sink, view, fireplace etc.).  Each room is thus allocated a rent band. Current rent bands can be found here

First and second years in college are charged nine weeks rent, second years outside college and third and fourth years are charged ten.  After that, rent is charged on a daily basis at vacation rent, which is pro rata of the weekly rent for the allocated room.