If you want windows that look great and open smoothly, complete replacement is definitely the way to go.
But if your main concern is air leakage and energy costs, consider storm windows.
Quality storm windows can stop air leaks almost as well as full replacements but cost half as much—or even less.
Plus, the labor savings are huge; a DIYer can install 10 storm windows in the time it takes to replace one window.
You might find inexpensive storm windows, but shop around.
Chances are, you'll discover that spending a little more is smarter.