Weiner adds, “Assuming a homeowner can reduce energy use for space heating by 10 – 25% by sealing ductwork, the average homeowner could save $40 - $100 per year.” It’s important to note that sealing ductwork not only saves energy during the heating season; if you use central AC, it will save you money during the cooling season, too. And thanks to the improved performance of the system, sealed ducts help reduce the workload on your (expensive) heating and cooling plants—that is, your furnace and AC unit. While this isn’t really a measurable factor, it’s worth noting that any mechanical equipment tends to last longer and require less maintenance when it’s working efficiently.

If you really want to know how much sealing your ducts will save, you can have your duct system tested by a home energy auditor or HVAC pro with special pressurizing equipment. Similar to a blower door test, duct testing equipment uses a fan to pressurize the duct network and detects leakage by how much air flow is present. By doing the test before and after you seal your ducts, an operator can calculate how much energy, and therefore money, you’ll save in an average year.

Source: http://www.networx.com/article/seal-your-ducts-to-save-energy