Pretty much everything in the centre of Oxford is expensive although there is a Youth Hostel there.
It rather depends on whether price, location or quality is most important to you.
If you want the cheaper ones there are a number of guest houses on the Ifley or Abingdon Road like this:
http://www.eurobookin...ford-guest-house.html
If you want to be really central and push the boat out (and can get in) there's the old parsonage
http://www.eurobookin...-parsonage-hotel.html
The four pillars is about a 10 minute walk from the centre
http://www.eurobookin...res-four-pillars.html
But there's not a great deal of pubs offering accoation close to the centre