negotiating cruise prices?

Told some people that a travel agent charged me the same for a cruise as the posted price on that cruise line’s website and in addition will give me a $600 rebate. They said this is not a big deal because you can usually negotiate with the cruise lines. I know this happens on those giant Carnival type cruise boats which evolve into a Priceline bidding phenomena, but I’m talking about a very high-end 270 passenger boat, where the lowest end cabin ( which we took) is already booked up for a cruise not leaving until September 2018.

Is it true that you can negotiate with the high-end cruise lines such as Regent and Ponant? and if so what , how does this take place and what can you expect.