Agree with Simon. I recently started using Cerberus, and, not only is it free, as opposed to kayako, but, in my opinion the support is great. There are always people browsing the forums that can answer your question within an hour and it will be an answer that will help you. You can also submit a ticket and get a useful reply within 24 hours.
I did not have any problems setting up Cerberus. There are a few config issues that came up, but they were quickly resolved via the forum, I don't even use the ticket system since all of the developers check the forums daily.
I highly recommend it. They are also going to be coming out with a CMS system that will be very handy and come at a great price. One of the dev guys mentioned $50/year for a license, one time fee, if you want support it'll be cheaper the next year. But thats not a concrete figure by any means. Just a very rough estimate.