Got this straight out of the oxford finance guide so should be right:
"What if I am studying for a second undergraduate degree?"
UK & EU nationals who have already completed an honours degree at a UK university, or have an equivalent qualification from an institution elsewhere in the EU, will not normally be eligible for the loans for tuition fees or maintenance, or for a government Maintenance Grant. Also, they will not receive an Oxford Opportunity Bursary. The university tuition fee for a second degree student has not been confirmed for 2008 entry onwards.
Students studying a second undergraduate degree will also be liable for college fees. These are payable annually alongside the university tuition fee, and vary by college. In 2007/8 they ranged from £4,400 to £5,400 per year. Please contact colleges for further information.
It's different for medicine.
The course fees also vary from £11,205 to £12,810 per year.
On top of this, you'd need to pay your accomodation, and living expenses, as well as society fees for the oxford union and any other 'clubs'. There's the potential for field trips etc too.
I'm averaging it to be (from information in the booklet):
Accomodation: £3,150 a year.
Food: £1,500 a year (eating in halls)
Course costs: £350
Social activities/clothes/personal items: £1,500
Transport: Low if own a bike. £500 a year if not.
Course: £12,000 average
College fees: £5,000 a year average
That's around £24,000 a year. So over a three year course, you're looking at £72,000.
Then there's also optional fees such as field trips, tv licence, stuff you buy FOR uni beforehand.
Not trying to put you off - just giving a reasonable estimate.