In good condition, both have been going up in price. I’d log on to Flea Bay, search for one, then click the sort box for Completed sales for a private sale value. Then I’d also check at the salvage yard to get retail price.
Are you buying or selling? Also, I think the rear windows go by hatch or coupe, not by LX/GT. That’s unless the GT came with different lettering, then you still have two shapes of rear windows. Please post what you find.
I have read about the places that can replace the seals on your existing glass being swamped, but that’s been a while. Also, I saw a reproduction that does not include the “Mustang” lettering for a good discount because of Ford licensing costs. Any place with a vinyl sticker cutter could make a label in white or several colors if you had to have the letters. But if you have to have the letters to still tell it is a Mustang, you might have done too many body modifications. :-$