The prices all work out the same, depending on what you personally prefer to spend your money on. But here's no way to calculate that. Each College has its own way of presenting costs, but the Bursars, who set costs, know that students have very similar (small) budgets.
Your chances of a double of 3/4 bed are slim to nil. Most rooms have singles, and you can't guarantee what accommodation you will get in any College anyway.