How much does this save? I took the 2014-06-18 data that backs my apartment price map and computed the average cost per bedroom for each number of bedrooms:

Perhaps apartments with more bedrooms tend to be in cheaper areas, though? I made six maps, with markers for all the apartments of that size:

It does look like living in a one bedroom is about twice as expensive than sharing a larger apartment, at least in Boston.

(These numbers all assume one person per bedroom. If you share a room with someone else then they're cut in half. Though a 4br or 5br is probably not going to work if filled with couples unless it has more than one bathroom.)