The entry fee was $1395 this year. Costs of tires and gas, etc will depend on the specific race car of course. Then there's the miscellaneous stuff like crew credentials ($30/each if memory serves), feeding everybody, getting a NASA license ($75?), practice day entry fee (Thursday is way cheaper than Friday).
NASA/Thunderhill want you to buy fuel at the track - at prices that will shock you. Their riff is that the extra $$$ you pay for gas goes into facility improvements, which is good for everybody. You can ante up or not. If you bring fuel into the track you're supposed to pay an extra fee. I'm not sure how much that is but last time I looked (in '09) it was significantly cheaper to bring your own and pay the "corkage" fee.
The motel rooms in Willows fill up for the event. There are other towns 20-45 minutes away (Orland, Chico) that generally have rooms available. Another alternative is to rent or borrow an RV and stay at the track. That's probably more expensive than a motel room, but it's real nice to have a warm place close by to retreat to for a nap in the small hours of Sunday morning.