The Oil Drum

I was quoted €6500 euros for a complete ground source heat pump system here in Germany, if installed as part of a new house build [1] so I could imagine laying the piping being around 1500GBP. Fertighaus's are largely made of chipboard and vast quantities of expanded polystyrene. The walls are typically around 40cm thick and filled mostly with insulation. 0.1 -> 0.14 W/m^2K heat flow.

The crucial point being that it is part of a new build, much more expensive to retrofit to an existing property. Plus it comes down to supply and demand, if you're the only one in your state trying to install an HP system it's going to be pricey.

You may well find it's cheaper to knock an old property down and put something like a fertighaus up in it's place rather than trying to heat and maintain a traditional property over the next 25 years.

[1] Fertighaus: http://www.fertighaus.de/