From what we learned it kind of depends on your floor plan. We have one 12,000 BTU unit in our downstairs (800 sq feet) and then we have 8,000 and a 6,000 BTU units in two bedrooms. One could have done the whole upstairs but we wanted the flexibility of being able to close doors if needed.

This ran us about $8-9000 last year. Ductworks was our installer and the equipment is all Carrier.