Taking your own car out on track can be very expensive, but there are a few things you can do to cut down the costs of a trackday, like buying a car specifically for the track & stripping the insides to kill off any unnecessary weight, and when on a open pit lane, try not to do more than 20 - 30 minutes at a time, come off & sit in the pitlane que. Your car like us gets tired and punishing it at high speed with sharp bends and turns is not the way to go!
Alternatively, buy track day tyres for your every day road car, instead of using your tyres for both the track and the road and you will find you'll get alot more use out of them!
These are just a few things I find in the long run cut down costing of my expensive habbit... For other hints and tips try joining a forum specifically for Track days, many pro's out there with more tips than me will always be able to give you some cost cutting advice!
Also never buy the first track day you see, the prices vary from as little as £99.00 to £179 in the uk (sometimes more including extra drivers), go to www.trackdays.co.uk as they have a list of all the suppliers track days in the UK, allowing you to compare the prices and spot the days where they charge between £0 - £35 pounds for an extra driver.
Hope this helps,
Tim