You could be eligible for a tax credit of up to $1500 for residential, or up to $1.80 per square foot, for commercial. Don't let the cost of installing a retrofit product keep you from reducing energy costs. Funding is available through legislation such as the American Recovery and Reinvestment Act.

Visit the links listed for the most updated information. 

Alliance to Save Energy 
National Association of State Energy Offices 
National Association of Energy Service Companies 
Energy Information Administration 
Energy Loan 
National Fenestration Rating Council 
US Department of Energy, 
Green Building Certification Institute, 
Database of State Incentives for Renewables & Efficiency