I would watch out with the payback estimators. FWIW - in my climate with my house, an energy star audit showed that geo would save about $300 a year on a total energy bill of $3500 (counts lights, hot water etc - about $1800 was HVAC). Were they right, I'll never know. But I suspect that everyone who has installed one thinks they are saving greater than $300 a year....

Comparables - Solar hot water $300, R 40 (from R 30) attic insulation $100, Seer 17 Hp - $250