There's a thread in here about warranty that sort of veers into the question of insurance too. You might want to read that before taking it to the track (another expense).
My rear PS2s are about shot after 4 track days and 16,000 miles. Stock tires are fine for the track (not best, but safe and easy to monitor) but you will wear through them faster.
My first weekend at the track the stock brakes were fine but the second weekend (with brand new stock brakes) I overheated them and they got stuck all over the rotors. Car vibrated horribly when braking. Some people here live with it and say the pad material eventually wears off the rotors. Brakes are harder to monitor and stock are not safe. I'd recommend not getting to the point I did and get track pads on the first or second trip to the track. Another expense right away.
You need a helmet too but I think you said you had that.
I just wanted to second what Dave wrote about the cost of taking the car to the track and give you an idea of what you need vs. what you want. Brakes = more of a need, tires = more of a want. It's a lot higher than just the cost of the event and Dave is right that what you need and thus the cost depends on how you drive.
BTW, it's also a whole lot of fun!
For that cost you generally get to spend 200 minutes over the course of a weekend driving on the track. If it's a BMW CCA event you'll get an instructor in the car with you to teach you to drive better/safer. The drag strip near me is about $20 for 14 seconds. Which would work out to $17,142.86 for the same 200 minutes of actual driving fun. But who would want to make 858 runs down a drag strip over two days? The track is expensive, but not that expensive and it's fun enough to do for more than 14 seconds!
Enjoy, whatever road you take.