The pay per clicks are similar to google adwords if you have ever delved into that? Essentially at the end of the day, if you generate a booking at a click through to your website at a cost of $2.25 for example, you are still coming out on top as opposed to booking.com etc generating the booking for you. Tripadvisor are also paying the google adword fees to get to the top of searches, so it is a pretty cheap alternative to adwords!