Hi
Our entire house is only 1800 sqft and we only have the geothermal pipes for the downstairs which is about 1000 sqft. Our borehole uses the water as I've said before in an open loop system which means the water is brought up using a pump and runs through the house and brought out and sent towards the lake close by. We have a very high water table and was told this was most suitable for us. To have a closed loop system means that the borehole needs to be very deep so that the earth can warm the water again on the way down, so that it is warm for the journey up. Our borehole is only 100 feet deep, however this is not the norm as far as I know. I believe that to heat a large house using a closed loop system the borehole needs to be 100 - 150 metres deep, so this is probably accurate. I think the going rate is approx. 28euro a metre. This is only for the drilling though.
Hope this helps.