In my experience it hasn't been very much. Maybe 5% which is why I've never purchased a new boat. I usually buy a 2 year old boat. I don't think the dealers have a ton of room to work with. Ironically, I felt like the best deal was a new bass cat which I see you own. Good luck!
I agree that it doesn't seem like dealers have a ton of room to work with. Only exception is if (like Donald said) you're bringing other customers to them and they're willing to break even on your boat to make it up to you. But that's not common.
My boat was bought new in 1997, so my experiences are mostly anecdotal from others.