Regularly changing your air filters (adhering to a regular, ample schedule) is an extremely inexpensive and easy way to lower your energy bill. By changing your air filter regularly, you are ensuring that your HVAC system can operate at its highest efficiency (and making sure your HVAC system stays in good working order), meaning it takes less energy to push air through your home.
The U.S. Department of Energy widely publishes the energy savings associated with regular air filter changes. They estimate that regularly-scheduled air filter changes will save you 5-15% on your heating and cooling costs each month! For most homeowners, this is a savings of $12-$25 every single month.