We just recently got Geo HVAC, for our two units it was 800 total feet divided into 2 wells. And drilling was in the area of 25% of the total cost. 50% for the units themselves, and I guess 25% for the other parts, labor, and misc stuff.

While drilling is definitely a big chunk, all of it is so much more expensive than traditional air heat pumps that even without drilling it would still be at least 50% more expensive. Double without US Federal incentives.