It depends on how you look at it. Windows aren't stocks, you don't purchase new windows with the expectation that somehow they will pay for themselves. One of the reasons many people buy windows, is for the same reason they purchase an automobile or put a roof on their house. As simple as it may seem, they may just want new windows because their old one are cold and drafty, in need of major repair or they are just are just tired of maintenence and upkeep.

So, here's another take on windows as an "investment". Suppose your home has 25 windows and costs $2500. per year to heat and cool and you decide to keep living with your old windows instead of purchasing new ones and assume the new windows would save 30% per year or $750. per year. That means after 20 years, you would still have the same old drafty windows and would have spent $15,000 more in utility costs than if you had replaced the old windows. But, that's not all. Lets say utility costs increase by as little as %5 per year. It's very unlikely it would be that little but lets go with that. Your total lost savings in 20 years would be almost $25,000. and you would still have the same old drafty window that would be much worse condition in 20 years than they are even today.

Now lets toss in a couple of paint jobs including scraping and primer on the old windows, some money spent futily trying to make them a little less drafty and you would probably top $35,000 spent and still have the same old windows. Then you decide to sell the house but the purchaser demands you do several thing like replacing the roof and the windows. let's say 20 years from now those $700 windows you could have purchased today have tripled in price so now it's going to cost

$53,000 for new windows
$35,000 for maintenence and upkeep
$88,000 total spent

and you lived with cold drafty windows on top of it all for 20 years. I believe the utility costs will rise at a much higher rate than the 5% we assumed but, I think this gives you some idea of why people replace their windows.

Vince, I have a couple of suggestions - Check into storm windows and spring bronze weatherstripping (kilian hardware.com or your local "real old fashioned" hardware store). Then $150 estimate per storm is what's typical in my area (greater Chicago). Also, since heat rises, make sure the floor of your attic is properly insulated. If you do those things, I think you'll get the most bang for your buck. Suggestion #2 is to post this question on the general forum at oldhouseweb.com. A number of old home owners that regularly post there have done the math and have stayed with their old windows plus the things I've mentioned.

Aesthetically, new windows do not look "right" in an old home.

skydawggy's post takes a lot of good factors into consideration, but does not consider the potential need to replace the replacements. Here is an excerpt of a post I wrote about my experience with original windows made from old-growth wood vs. modern windows:
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I should also say that my house is 120 years old, and I have no plans to replace any of the original windows, which I am slowly restoring by hand, with many thanks to informative posts on this and the old house forum. The only reason I need any new windows was due to a poor "remodeling" done 40 years ago, where some original windows were replaced with "modern" Andersons. The 40 y.o. windows are all rotting and leaking; the 120 y.o. windows, except for needing a little reglazing and sash cord tlc, are going strong.

I don't expect any modern window to last more than 40 years, which is why I'm keeping all the old ones.
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If your replacement windows last as long as the originals, then you're set. If your future buyer prefers new windows to originals, then you're fine too. But if not...

What I read over and over again is, reglaze the originals, get insulating cellular shades, curtains, harvey triple-track storm windows, spring bronze weatherstripping, and old windows will perform comparably to new ones in terms of heat loss. We are in the process of doing all of these things and we've had favorable results so far, for a lot less money than replacements.

We were looking at two houses on the same street for sale. One had original windows and other original details. The other had cheap replacements and other original flooring and woodwork ripped out. Guess which one we bought.

The older your house gets, the more valuable its original parts will become.

One more thing, if you're not too far from Boston, the non-profit Boston Building Materials resource center might have competitive pricing on some good storms:

Here is a link that might be useful: source for Harvey tru-channel storm windows