UNITE was pretty cheap when I was there as I recall, they have several different locations, one is fairly close on Blackboy Road, but even the one in the city centre (Northernhay) isn't that far from St Lukes. It'd take you maybe 10-15 mins, tops, to walk from the city centre to the St Lukes campus. Alternately you could look for private rental options in the Heavitree area, which will likely be cheaper and there are usually some student lets around there (it's where we lived after first year, even though we were based on the main campus).