Almost Every Window Retrofit Option Offers a Better Return on Investment than Replacement Windows.
Findings from the cost analysis sponsored by the National Trust for Historic Preservation showed that new, "high performance" windows are by far the most expensive measure, costing at least double that of common retrofit options when considering materials, installation and general construction commonly required for an existing home. In all climate zones analyzed in the study, cellular shades, interior storm panels and various exterior storm window configurations offer a higher average return on investment compared to new, efficient replacement windows.
For more information see:
The National Trust Resource Library
The California Office of Historic Preservation