It isn't cheap if something goes wrong..
You are liable for all 3rd parties if you drop oil/coolant, so if your not insured and leak some coolant which drops a biker who incurs injuries as a result then your up shit creek without a paddle.
Barrier charges, recover charges are a drop in the ocean if the above happens but they aren't cheap either.
20k is a tad OTT but make sure you take a credit card with at least £3k spare to cover any barrier damage or track closure fees if the worst should happen.
Many people, myself included have been .. had a great time and come home with very little drama. I went in my RX7 and a friend went in a TVR we both got there and back in one piece without so much as a sniff of trouble both mechanically or by running of talent on the 'Ring.
I can't wait to go again!