Your question isn't too clear to me, but I make my own bookings with the cruise lines as an associate agent. I often go to one of the big on-line cruise booking sites and go through the motions of making a booking just to see what the ballpark cost will be before calling. These days, there isn't much difference (if any) between an internet booking or one offered by an agent - except that the agent will treat you better and look out for your interests.
Most often, the fares for 3rd and 4th guests you see on-line will be the same as what the cruise line is offering for that particular cruise regardless of where the booking comes from. I don't pay much attention to the brochure rates.