A ballpark estimate is difficult without additional information but I can tell you what I was quoted for my house. I have a 3000 sq ft house with 3 floors. (well 2 floors with a finished rooms in the attic) To replace my 17 year old geothermal unit in the basement (without replacing the pump, ground loop or other piping) was 8k, and the most efficient unit was 10k. Keep in mind this is what is required for just the first floor, I would need a 2nd unit to service the 2nd floor, proably running around the same price range. I currently have a heat pump for the 2nd floor and last months electric bill (Dec 2007) was $288 (all electric house). I'm estimating it will cost me $300 a month to heat the house this winter. I haven't lived there long enough to see what the summer bills are like. Not too bad considering I was paying about $150 for Natural gas and $50 for electric a month during the winter on a 800 sq ft house before.

Another thing you need to consider is if each apartment wants to control its own tempature setting, it will require a seperate unit for each apartment.