what you pay depends on your risk tolerance, how much you know, how much you care, and how much you do yourself. I'm sure I'm forgetting stuff, but there's
- the fee you pay to the organizer for the event
- track insurance if you want it (for me this has effectively doubled the entry fee)
- for some events you may elect to stay in a hotel
then there are the costs incurred around the track days. these include things like
- inspecting the suspension
- flushing brake fluid
- brake rotors and pads
- changing the engine oil more often
- changing the transmission oil more often
For me, all things considered I'm guessing the total cost of an event comes out somewhere between $700 - $1000, and I'm doing all the work myself. Admittedly it could be done cheaper (maybe closer to $500?) but you don't want to go too cheap. The last thing you want is to have the car fail while you're zinging around the track at max speed.
"Give a man a match, and he'll be warm for a minute, but set him on fire, and he'll be warm for the rest of his life."