Personally I think upgrading to newer double paned windows is a definite plus. BUT, I am not one to want to live in a cave with curtains closed all winter to keep the cold out or closed in summer to keep the heat out.
I had every window and door replaced in my home with double hung double paned low E and have noticed a decrease in the electric bill. Not to mention no drafts.
You certainly should not have to spend 6 grand for the amount of windows that you need. You can get stock sizes at any home improvement stores and replace yourself. You may need to enlarge the window opening.
Alot of cost from a window contractor comes from the "possible" rot repairs that will be found once they open up and start replacing your windows/doors.
In our repair forum you will find many threads that are about window replacements, don't forget to check out the pics of many as well.
The difference between success and failure is who gives up first!