I'm not much of a structural expert and wasn't even aware that window frames added anything to a buildings support! I would have thought builders would design houses to be structurally safe with no frames! Anyway, since I used to work for a double glazing firm here's my two- penneth worth.
In 2003 I ordered 11 windows from my company (2 x double doors toughened glass, 1 bay window consisting of 5 separate frames, 3 top openers and two bottom openers all using toughened glass as it was close to the road), the remaining windows were a menagerie of different sized typical house windows. All in brown.
All were approved (as your purchase should be) to Document L standard (introduced sometime in 2001-2 I think). Brown was extra cost. And the double doors should have retailed at =A34000 each! Total bill in excess of =A317,000. I paid =A33335 (including VAT). And the build cost was a mere =A31200. For the lot. I checked. I worked there so looked up the material cost.
My advice - whatever you are quoted - laugh, half it and then call them on the last day of the month when their desperate to hit their targets and knock another 10% off. Good luck