Have you had an energy audit done on your current windows and home? Usually they will give a report indicating the ROI of the different things you can do to improve the energy efficiancy of your home.
You can also compare the ER (Energy Rating) of the windows. Although I think ER is a Canadian thing, they should have something similar in the US. Depending on your climate you could have a payback period of several years for the good windows. Or for the cheap ones you continuously spend money as your heating or cooling energy flows out them.
A good site for some tips on picking windows is here "http://www.gov.mb.ca/conservation/r2000/windows_tips3.html". It is a website of the Manitoba government here in Canada.