Here in the DFW Metroplex we have a handful of companies that advertise replacement windows on the radio. From my recollection they typically advertise between $300 to $600 installed per window depending on the type (e.g. single-double-triple pane, single-double hung, etc.)

I had a neighbor recently replace there's and all of our homes are about 15 years old. I didn't really understand why he would do it (I haven't brought up the subject), as my house is of similar size and on average my electric bill as about $100/mo. I haven't counted my windows, but let's say I had 30 of them. At a minimum of $300/window (would probably be more in reality), replacement cost would be $9,000. Let's say doing this halved my electric bill (which it would not), that's a $50/mo savings. $9,000/$50 = 180 month return on investment. In other words, it would take a minimum of 15 years for the windows to pay for themselves, but in reality it would probably be at least 30 years or more in my situation.