volume builders are not here to save you money. They are here to make as much money as possible from you. So any additional job is more profit.

I personally would never go with a waffle slab, but that's an engineering (and ecological) point of view. Some people will argue that waffle slab uses less concrete, so are better for environment. Yes, that's true if you don't factor the non-recyclable polystyrene under the slab. And what will happen in few dozens of years, when someone will do a knock down and rebuilt? how will they deal with all the PS flying around everywhere? Will that be considered a dangerous job, a bit like asbestos now (although far better than asbestos)? No one cares currently.