I've always found that when I book directly with Carnival they give me the same price as a travel agent.
Again, it depends on your travel agent and the kind of service they provide, the price will most likely be the same, but a good, personal travel agent who cares and takes the time to go over everything, and is more like a friend than one who just wants the money, is priceless in my opinion. Another plus is they can take care of all your travel needs, all cruise lines, hotels, air, etc.
I've really enjoyed dealing directly with Tamara Matulonis at Carnival, and would most certainly recommend her to anyone thinking of a Carnival cruise.