[dlcarrier]

Going from 8 ft to 9 ft ceilings adds 12.5% more volume. For an 1,800 square foot house, in a 30x60 ft form factor, that increases the surface area exposed to outside air by 5.55%. This would create a small increase in the heating and cooling price per square foot, but a similar decrease in the cost per cubic foot.

[bluGill]

You missed something important: many people are moving from a much older house with poor insulation. They can double their surface area and yet see their HVAC bills go down by a lot because modern houses are so much better.