Well, although we have not yet moved into our house we have had 2 heatpumps fitted.
At the moment we are living in our apartment which was new last year with a heatpump installed, so we have had the use of the heatpump for the whole of the seasons. What you must remember is we are in Auckland so do not get the cold of Christchurch but we have found the heatpump to be extremely efficient.
In the winter we only needed it on for a couple of hours in the morning and evening because the insulation is very good so the heat is held in the apartment and our highest heating bill for a 3 month period was $225. This is a small apartment so obviously bigger places will cost more.
we have the air con on for hour bursts 2 or 3 times a day and this keeps the place cool. we keep the windows shut as we are by a pond and there are so many flys but because of the good insulation it stays cool longer.
Our new house is well insulated and double glazed so hopefully we can keep the trend going there but it will cost more being a bigger place.
So I would say as you are building your own place then with good insulation and double glazing you should be able to run 1 or 2 heat pumps which will be fairly ecconomical