Agreed...$26K is definately high based on the openings you mention. My advice is to get some more quotes on other "high end" triple pane windows; Simonton 9800, Alside Preservations, Sunrise Restorations, etc... You'll get a pretty good idea what the "going rate" for a high end window should be in your area.
If you still feel the Schuco is the best window after you've done your shopping you'll have a lot better bargaining leverage with NES. Make them an offer...."Based on my shopping, the best price I can justify is $X."
I'll bet they either take it, or drop significantly from your $26K quote. Just a hunch...
As for the alleged monopoly you mention; it really isn't that way. NES (and all other Schuco dealers) do have competitors and therefore they don't have monopoly pricing power. The customers just have to do their own homework as well to make sure they get the best deal possible. Very few people when they buy a new car take a look at the sticker and pay that amount....