It only makes financial sense to replace windows in a house when the surrounding frame structure is rotted or so badly shrunk, re-caulking won't help stop all the drafts. Changing from single pane to double pane glass is good for noise abatement but leaking is generally from the seals at the frame interfaces.
Three of my ROMEO buddies wives have talked them into expensive window replacements ($30 K +) in 20 year old houses because that's what other wives are bragging about at the Garden Club luncheon meetings.
Even with leaking windows, payback periods are measured in decades. Gumby's post is right on.