I don't know much about this, but anecdotal info: my apartment (comparable to a 1-floor bungalow with lots of windows) has argon-filled windows, and it cost me around USD$1600 for the window themselves (I didn't change the frames, but my old windows were cheap/scrap).

I heat to 18oC in winter (Montreal) and AC to 24oC in summer. My electricity bill averages USD$40 per month. The windows made a nice difference, and still seem efficient after 10 years.