Insurance is not 100% necessary as you and everyone else sign an indemnity which means you can not claim against the track owners or owners of other vehicles. You are basically there at your own risk.

You can cover yourself / vehicle shell with insurance if you think its worth it, but track experience usually makes no difference to the price.

My trailer was £1000 plus vat.

Average winter prices are around £120 for the day
Average summer prices are £180 for the day.

HTH