I live in a 1959 ranch-style house with the original windows. They're the aluminum frame with single-pane glass and very poor weatherstripping. When I moved here, there was maybe r4 batt style insulation in the attic. The first winter I was here, I had heating bills nearing $500 every month. That spring I had loose fill blown in the attic to a depth of 24". It made such a profound difference that the A/C was massively oversized in the summer. The following winter, my energy bill dropped by 2/3 of what it was the previous year. A couple years ago I had the HVAC system replaced and went from a 5 ton unit to a 3 ton (which is still oversized) and halved the bill yet again.

To summarize: I'd be far more worried about the roof insulation rather than the windows.

The attic insulation was relatively cheap and made a profound enough difference in the energy bill that it isn't cost effective to replace the windows. My utility bills are already low enough that I'd never recover any cost, or if I did, it would be over such a long period of time that I probably wouldn't live to see the break-even point.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)