Creating a profile and messaging TripLeaders is completely free for TripMates. As soon as you have found the right Trip, you contact the TripLeader.
When he has confirmed your spot on the trip, you pay him to cover the price of the trip. The first payment is for 20% of the price and shortly before departure, you pay the remaining 80%. The price includes the cost of transport and accommodation at your destination, not transportation to the starting and endpoints of the trip.
Depending on the trip, local activities may also be included in the price. The exact costs can be listed in the description of the trip. Otherwise, just ask your TripLeader directly. He'll be happy to help!