try this site:
www.dailyinfo.co.uk
(the most useful site for buying, selling, renting, letting, etc ... in Oxford)
- in addition to browsing the notices and ads on available accomodation, you can also place a free ''room wanted'' notice at
www.dailyinfo.co.uk/unified_adboard.php?col=rw
there are a few other "cheap" hotels around Peartree interchange on the A34 and Oxford Business Park (Premier Travel Inn, and Travelodge ... if I recall correctly); there are also quite a few B&B/Guesthouses on Abingdon Rd (very close to the centre, one of these cost me about £40/night a couple of years ago)