www.hvacopcost.com

Gives a good calculator for heating and cooling cost calculations

For me, the difference between a 80% and a 92% efficient gas furnace is
only about $14 a year

Cooling for me going from 8 seer to 13 seer, I see a drop of $800 in
cooling costs.

In the mountains of NC the difference between a 80% and a 95% gas
furnace is only about $140 a year.

going from an 8 SEER AC to a 13 SEER AC will save about $400 a year.

These figures assume a 1200 sq ft house, with normal insulation. You
can plug in house size, SEER and furnace efficiency ratings on the web site.