Cost of accomodation, fuel, food, a few laps of the Ring and train/ferry fare are likely to run to £500 - £600 quid.
I did about 10 laps in my 1996 M3 last year and didn't need tyres, but I did kill the clutch, gearbox, diff seal, 1 bearing and 1 wishbone along with a pair of front brake pads. You can judge by that what you will, but I would budget at least 2-500 quid for parts/tyres/emergencies depending on what you drive.
Insurance will run you around 1% of your cars value per day, usually with a hefty excess around 5-10,000 quid depending on the cars value. Most people say it's not worth it as a result.
It's a very hit and miss logic deciding whether or not to drive on the Ring without a 'specific' track day policy for the track itself. Your insurance company 'probably' won't cover you for it, but they probably won't or can't legally exclude it either, as it's a public toll road.
Whether you insure your car in advance or not, if you crash on the Ring you are going to need deep pockets.
I'm not trying to scare you off - I just did it without any insurance and was prepared to bear the cost if I damaged either the car or the track, or anyone else I suppose!! I'm not sure what to do about the Golf this time, I might be willing to look into a 'group buy' on insurance if enough people are interested. I'm sure an organised group will be far more attractive to the underwriter and provide more revenue and less risk!