@turquoise-- Yea, it is kind of expensive. But you don't have to pay a lot for tea tree oil shampoo. Just buy pure tea tree oil and add some to your regular shampoo. People do this all the time and it works just fine.
Make sure you do a skin test with tea tree oil first though because some people are very sensitive or allergic to it. It can cause skin irritation and rash if you're sensitive to it.