of between 2 and 20 years, based on energy savings. That's without taking into account things like a smaller heating system and other savings.
As a simple rule, it is almost always worth forking out extra cash for better windows. Not only do they give you immediate comfort gains but they will pay for themselves over time in reduced energy costs, less maintenance and reduced hassle.
If you really want to go the whole hog, super-high-performance windows can cost you a thousand dollars (or a couple of thousand) more for the entire house. They will be multiple-glazed (double or triple), with krypton gas filler and multiple low-e coatings. These windows come in at about R-7 overall (an amazing R-12 in the centre of the glass), which places them about 3 points higher than the best standard energy-efficient windows.