Main costs are ...
Tyres £300 a corner and seem to go through them quick but I guess this depends on how you drive!
Budget for winter tyres too if you intend to use it in the winter.
Servicing (not too bad but the 30K one is around £850).
Brake pads weren't due on my car until around 48K miles (not sure how much that may cost)
If you are looking at a 2007/2008 car you will need extended warranty (Comprehensive cover is £87/month)
Insurance
Road Tax
Fuel (lots of it the M3 has a drink problem
)