How do sash replacements compare in price, installation and utility compared to new replacement windows if my old window frames are in good shape?
What type of replacement windows are you planing on using, what type of sash packs? more info would be needed to answer properly.
Keep in mind those kits are the guts of a normal window, just without the frame parts. Expect that the savings will be in installation and [lack of] need for trim/other materials. The actual cost savings kit vs. window will not be tremendous.