When we bough the house three years ago we knew that the windows needed replacing. They were old Everest aluminium framed double glazed units. They had many problems: the metal frames conducted heat like mad and had deformed over the years, several of the units had blown and none of the glass was modern Low-E.
New windows are expensive however you look at it. I looked at quotes from Internorm and The Green Building Store for modern triple glazed units, using modern glass, spacers and cavity gas. For high performance "A" rated windows it's between £20 - 30 thousand, fully installed.
Even assuming high gas inflation of 10% (which is plausible) our house doesn't lose enough heat through the windows to break even until over 35 years and assuming 35% heat loss through the windows - which is more than it really is. Basically expensive windows will not pay for themselves on gas prices alone.
If you have the money required to buy the windows in the bank at the moment, and just sat on it, it would gradually devalue over time as bank interest is consistently less than inflation. If you factor in a 2% difference between interest and inflation against you, then overtime you lose a lot of money in a bank savings account. If you add this to the cost of domestic gas for heating then the break-even point is now at more realistic heat loss through the windows percentage and/or date.
It's still not a really strong economic argument for new triple glazed windows, but as the ones that came with the house when we bought it had failed, this summer we decided to buy UK made Green Building Store Ecoplus 3 windows. By making the windows simpler than the original ones we were able to save several thousand pounds on the quote and bring the price closer to 20k than 30k.
After our holidays they came and ripped the old windows out and put the new ones in. They look an awful lot nicer than the original ones and let in considerably more light. We're now in the process of making good the mess that was made getting the old windows out and the new ones in. This winter I think we will have gone from some of the worst windows and doors in the village to the best.