RCL doe not allow TA's or anyone to discount their cruises. You could possibly find a TA who is holding group space on the particular cruise you are looking at and that could be cheaper than the rest.
Some TA's particularly online ones, will kick back some of their commission to give you OBC but that will depend on the cost of your cruise in the first place. If the price goes down, it is normal for them to reduce the OBC accordingly.
The cruiselines pay TA's a commission. There should be no charge to the client.
I am a TA - I do both brick & mortor and online. (Just to qualify what I'm saying.)
There are several questions to ask before entering into an agreement with a TA.
Do they charge a service fee? Is there a cancellation fee? Do they charge to make changes? Will they get any price reductions that you may qualify for? Are they reachable at odd hours rather than just 9-5?
What kind of a reputation does the company have? Are they solvent? Will all payments go directly to the cruiseline and not themselves first?
Where do you live? That can make a difference too. For example (horn toot here) no matter where you live (Canada, US, UK) if you buy through a TA who has their office in Ontario, you have added protection of TICO. Quebec has a similar program.
Any number of people prefer to book directly with the cruiselines. That does work I suppose if you know exactly what you want but they will not watch for price reductions, you probably won't get the same person twice and if you want help with anything but the cruise - they can't do it. If you need air they can only sell you cruiseline air (which most of the time sucks). If that's not available you are on your own. If you want a hotel either you get their very expense pre or you are on your own.
I think I've said enough for now.