There is no guarantee of "Best price" anywhere. Sometimes, what you get for a lower fare, you may sacrifice in service. Find a price that is fair, an agency that is reputable and don't worry that you are getting the best "deal" out there. Sometimes deals aren't such good deals after all.
There is a general price that cabins are often at from the cruiseline. Any prices below that price are usually a result of the travel agent refunded part of their commission back to the client in the form of a reduced fare. Can you get TAs into a price war over you? Yes. Will that guarantee you good service? No. I'd rather have a TA that gives me a fair price upfront and doesn't play games. If that means paying a little more because I don't play the agents against each other, then so be it.