Expense is relative, and there is no alternative to compare with for this kind of upholstery. Blown fiber, goose down, etc. won't hold shapes like poly foam. I guess you could use horse hair...
I'd say that the Blu Dot sofa is all foam. It could have some kind of low-profile seat springs (sinuous wire) or elastic webbing, but that is difficult to gauge from a photo.
You can make seat cushions with springs inside
them, like this