Most track insurance premiums cost approx 10% of the full replacement value of your car for the day, on top of this there is usually a £1,000 excess, so if in a £30K car looking at £300 premium for the day. If you are unfortunate and write the car off you will then recieve £29K.
To keep it cost effective I tend to ascess the risks associated with the particular track and ask for a specific amount of cover, usually £10k worth and this costs me about £80.
At the end of the day you are taking a risk with your pride and joy but its worth it IMO and great fun, our roads can be far more dangerous.
Bedford Autodrome on the other hand is one of the safest circuits in the UK so if at this location I don't bother with cover as very little to hit, apart from other cars of course !!
It all sounds very risky but you take a risk everytime you get behind the wheel, give it a try certainly teaches you an awful lot about your car and your own abilities.