It depends on how much the track time itself costs as well as how far away the track is. The closest track to me is 6 hours away, so gas costs are a big part of my typical trackday expenses. In total, if I go to Barber in Birmingham (one of the best tracks in the country, even though it is 7 hours away) for two days, I will spend roughly $1,000. The track time is $400 ($200 per day), plus tires ($300-350) plus gas (~$190 for the car and the bike) and snacks for the trip. I camp at the track to save $60 per night of hotel costs, and the organizers provide snacks and drinks during the trackdays, plus a free lunch. However, if I go to Hallett in Tulsa, Oklahoma, the tracktime is only $100 per day, and gas is a little bit less, since Hallett is a little closer.
You can get a good set of leathers for $300. That's how much my current AGV Forza suit cost me, and it was brand new. Sometimes you can find better deals on used leathers on eBay, sometimes not. Helmet, gloves and boots depends on your preferences but can range from $200-$700 (helmet) $50-$200 (gloves) and $100-$300+ (boots).
There is more, but I think you're starting to get the idea. The initial costs is pretty high for trackdays. The ongoing costs depend on how often you get to go and how far away the tracks are.
EDIT: I forgot to mention how much you will save on speeding tickets and insurance rate hikes once you go to the track. After riding at the track, you start to notice just how dangerous the streets really are. You will also notice that riding on the streets is a lot more boring...