I would never pay a booking fee to a TA. If the commission they receive from the cruise line isn't sufficient then they need to be in a different line of work.
In fact my TA has always been able to give me the exact same price as what I could get booking direct from Carnival or any other cruise line. That is why I use them, it's the same person on the other end of the line whether I'm booking a Carnival cruise, a week at Disney World, or an all inclusive.