I did some research recently as I too am looking at an M3.
I'm lucky in that my local indy specialist has a very good labour rate and I provide all fluids and parts for servicing so based on that:
Tyres
Rear: £159 each (assuming F1 AS2)
Front: £128 each (assuming F1 AS2)
Brake discs:
Full set of Pagidt: £540
Brake pads:
Full set of Pagid + wear sensors: £167
^Prices taken off ECP, free next day delivery etc.
Stock up on oil when ECP have their usual 30-40% off and supply to indy when needed
Tax
Band K = 1 May 2001 - 23 Mar 2006 = £285.00
band M = 23 Mar 2006 - 28 Aug 2006 = £500.00
(Glad I'm looking at a 2005
)
Insurance and servicing costs all come down to your individual specs. I have a good indy so everything is in hand and insurance is fairly cheap too so not really a worry there. I do maybe 3000 miles a year so I won't need to pony up for tyres and brake parts for very long stretches. Realistically the biggest annual cost will be tax with an oil service next in line.
This all assumes nothing breaks in between of course
The subframe was new news to me recently, I'd heard of it before but never looked into it. If your M3 is a 2003 then it's out of warranty with BMW so you can expect to pay £1000-£1500 to get the re-enforcement work done either via BMW or at somewhere like Redish Motorsport.
As it's not a daily driver neither of us will likely need to have the suspension coils seen to every few years as they do crack like clockwork as do the bushings wear. Those are all relatively cheap to replace though.
All things relative it isn't going to cost me a great deal more to run year round than my current E46