As expected they do not have a fixed price and varies greatly from one season to another, depending on whether the demand goes up or down.
The previous day, check the weather forecast before making the reservation, and if it is not good it is better to postpone it for another day. Be cautious.
Obviously, rental includes the driver and is not very expensive, especially if the expenses are shared among several friends.
You can hire them directly off the beach or at one of the travel agencies to be found in the coastal locality. The first choice would be to negotiate the price directly with the boatman, and that may save you a few baht. In any case, it has its charm.