Well, for starters the tuition fees are the same as at other universities.
Secondly, if you qualify for maintenance grant, you might be entitled to Oxford Opportunity Bursary, which is around £4000 in your first year and £3000 in remaining two. Compare this to, for example, KCL bursary, which I think is around £1500. So even if it is slightly more expensive to study at Oxford (which I really don't think it is, especially compared to London), you will essentially get more support then at other universities.
I don't know about Cambridge, but I would guess it's similar to Oxford in that respect.