Mr. Moleman is definitly correct on this.
A big part of the cost is getting all of the proper safety gear. If you don't have any, it can be expensive considering that you and your buddies can't just buy ONE race suit and share it, I believe you need a minimum of 3 for refueling purposes. Helmets are also not cheap. Expect 150 dollars for even the cheap ones. Then you need a race seat and harness, new tires, new brake pads, a roll cage (if you can build it yourself you save a lot of money, but you still have to build it correctly)
Then you have all the fuel you're gonna burn during the race, which if you're good, is a lot. I've gone through all the costs for a brand new team with no equipment, but ability to create their own cage, and you're looking at around 5-6000 dollars to field a car.
This number goes way up if you can't make a cage on your own, have to rent a trailer to get your car there, buy replacement parts, get high quality...anything.
It's not cheap, but it still is one of the cheapest forms of car racing out there for sure.