All rooms in Durham (except those mentioned) cost the same, shared or not. This creates an equality between students and prevents ghettoisation of either rich or poor students. Differential room rates are also impractical in an ancient university like Durham - I mean, how can you say whether someone should pay more for a large, but drafty and shared room in the Keep of the Castle, or for a tiny, but warm and single room in Moatside? Should Bailey colleges charge more because of their better location w.r.t town, or less because the science site is further away?
Evaluating the rooms is nigh on impossible, so charging everybody the same and allocating to firstyears at random, and to thirdyears according to a ballot is the only really fair way of doing it.