Sorry, what is your game plan? To purchase the glass yourself to get around the PB's price, and then, what, ask the plaster subcontractor to toss the glass into his mixer right before he shoots the pool? Have you discussed this with the PB, or the subcontractor, and gotten their blessing? Or are you just planning to wing it on plaster day?
I don't condone the crazy up-charges PBs use to turn their loss-leader base price into a profitable job. But that seems to be how the industry works. More to the point, if you're planning this end-run without prearranging this with the PB, you'll likely alienate him and/or possibly void your finish warranty. And that's if the sub agrees, which I doubt, because he won't jeopardize his relationship with the PB just to save you money, that's his bread and butter. And pardon me if I have this all wrong. I'm not judging, just cautioning you to be careful how you go about this, and keep your relationship and warranty intact with the PB.