I've seen discounts vary anywhere from 5-30% just in the short amount of energy I've looked at new boats (usually in the 35-38' range). It depends on so many variables, month, volume, model popularity, inventory hang, what options/margin it has, geography, whether they are a marina (many will toss in a slip,
storage for the first year instead of play with real $s), full moon, etc.
I can't speak for others of course, but if I got a 15% discount and lots of little goodies thrown in - slip, first year winterizing/storage, maybe free equip package. I would consider it a deal. I'm guessing their retail margins are 50-60%, but it's most definitely a very low volume, high margin business where there overhead is high. I don't begrudge them staying in business. I do begrudge them scalping me for minor equip add so negotiate hard on that aspect.
Good to at least know the model "boat show special price" to get an idea...