Argon gas windows can increase energy savings, but the difference may not be significant enough to be worth their high upfront cost. In general, replacing windows only for the sake of energy efficiency is not worth it, and if windows need replacing, insulated glass may be a better option.
Ultimately, the benefit that a home can see in energy savings with argon gas windows depends on its existing energy performance rating, which varies between homes and between climates. Energy.gov gives detailed information on determining a home's performance rating.
Some areas offer rebates or tax incentives for upgrading to energy-efficient windows that can help recoup the cost more quickly. Tax credits, rebates and savings are available by state at Energy.gov.
Another alternative is krypton gas windows, which cost more than argon windows but offer greater insulation. Some manufacturers combine argon and krypton to minimize the cost.