Welcome to cruise-chat.
You are not going to get a cheaper price booking directly with the cruise line. The price offered by an agent (unless they have a service charge) will not be different. In fact, Carnival, Royal Caribbean, Celebrity, and Norwegian Cruise Line all have policies which restrict any discounting via commission rebates or by several other enticements.
Actually an agent might have blocked out group space for a cruise, and in that case they can offer special pricing and perks which the cruise line itself cannot offer if you book with them.
Just remember that you don't pay the commission, the cruise line does. But your price is the same. So for example, if I pay Carnival $700 for a cruise I booked online with them, they keep it all. The same cruise can be had for $700 from my agent, but then she gets her commission percentage from Carnival, which comes out of that $700.