Well, yes, but ...
I do appreciate the issues of technical vs pricing being different. I replaced a Marvin window last summer. The new unit was a very nice unit with aluminum cladding, casements on each side, and a central fixed window. The old window installation had been faulty, and I needed to rebuild the supporting wood beneath which had rotted away. It was a great learning project.
Part of DIY work, though, is seeing if it really makes sense to do something in the first place. It took a couple of weeks to do the work, and there was comfort in knowing I had put it together well. However, if the discount the contractor is such that a pro could put it in for the same price that I would pay just for the window unit it really doesn't make too much sense.
In addition, if I knew the pricing structure and I were to buy a dozen units from Marvin I could save enough money to do what everyone here really wants to do, which is to buy more tools.