Ok, I finally dialed in approximately correct figure to www.hvacopcost.com

Keep in mind, I am in a Humid Semi-Tropical climate. Cooling costs
dominate, heating is trivial in cost by comparison

The difference between 80% and 92% AFUE is LESS THAN $25 ANNUALLY, in 20
years estimated life, only about $750 saved.

Their estimate of my cost is still about 50% above what I actually spend
on heat in a typical year here, so I scaled the savings down as per
their ratio of reduction.

On the other hand, getting a geothermal heat pump will save over $400 a
year Payback period is SHORT on that one. Only issue here is that we
need either several deep wells (400+ feet, 24 inch diameter or larger)
or must dispose of the well water on the surface, thru sprinklers, into
a newly dug leaky tank (pond to non-Texans), or into the storm drainage
ditch. Getting a well water permit inside the city COULD be a BIG
challenge.

For the NEW house, 12 SEER vs geothermal heat pump (20 SEER) has an
annual savings of over $1500, and the value of the home is estimated to
go up by $30,000. In practice, because this new home will have 9 inch
insulation in the walls and about 12 inches in the ceiling, will be
surrounded by trees that BLOCK the late afternoon sun, the actual costs
won't be much more than what I am paying now for 12 SEER. Now 1200 Sq
ft, new home will be 3556 sq ft of conditioned space. This is outside a
municipal water district, so I MUST dig a well anyway, just install a
larger pump than would otherwise be needed and sprinkle the water over a
250x200 foot area at a rate of about 0.48 gal per square foot per hour
at the wettest part, and in practice, each area gets wetted only once or
twice in a 6 hour span.

Heating costs for the new home will rise proportionally to floor space
but again due to the extreme insulation, not as fast a rise as the
increase in floor space represents