Cary,
I have an 82 SC; typically we do 12k a year in her mixed driving from weekends away to business trips up and down the motorway.
For running costs over a year I budget for one service usually between £350 and £500 at a good independent (although it has been as high as 1k!).
In addition to the service I change the oil at 5-6k miles my self costing about £70 quid (depends on what oil you put in)
The clutch was replaced about 30k ago and while the box was out I had first and third dog teeth replaced this cost about £550, the clutch is still going strong.
I change the gear box and brake fluid very 2 years and this takes about 2 hours and £50 pounds for oil and fluid.
I have had the front wings replaced but I think if I had looked at more cars when I bought it I could had avoided this, I was actually looking for an honest car i.e. very few body repairs and with all the warts showing so that I could restore it. I was surprised at the cost of the wings at over £400 each (a 996 wing only costs £150?)
This year has been a very expensive year in terms of running costs as I have had to replace the cooling fan, and oil cooler costing about £800 (no labour as I did these my self again)
Typically the tyres last about 20k and cost £100 for the rears (I run bridgestone SO3's
The only other surprise I had was when I replaced the plug leads, they are shielded in metal braiding and cost £150
The reason I bought the car was that it is really simple and fun to work on. So typically my greatest expenditure has been on "restoration items". Unless already replaced you can count on most of the rubbers and seals will need replacing. The problem with most SC's is that they are of an age that they are quiet cheap and being very reliable often get neglected and take some time and effort to get back up to scratch. Find a car that has been cosseted by enthusiast as we often spend more than we really should.
I've often thought that I could run a new car for the same amount I have spent "restoring" mine, however I know if I had bought a dearer car and hunted better then the SC would have been much cheaper to run and it is very much a 911. I live in Oxfordshire and if you would like a drive give me a call or you want to chat you will find my number in PP under the SC assistant.