The costs people forget
The entry fee is the number everyone quotes. Add tire and pad wear, track fuel and the drive there, and the real number is a lot higher. On a heavy, powerful car it can be several times the entry fee. Tires and pads wear many times faster on track than on the street, and fuel economy drops to a fraction of what you see on the highway. Spreading those costs over how many days each part lasts gives you an honest per-day number.
How to estimate wear
Track how many days your last set of pads and tires lasted, or ask owners of the same car on the same tire. Heavier, more powerful cars eat consumables much faster; a light car on 200-treadwear tires can get a season out of a set that a heavy car finishes in a weekend.
Insurance
Most street car policies exclude track events, including instructed HPDE days. Track-day insurance is sold per event or per season. It's priced from the car's agreed value, usually with a 10–15% deductible. If you skip it, you're self-insuring the car, so count that risk honestly.