Leaving aside all the ecological and moral arguments for a moment. The most asked question is: How much do I have to spend to get a GSHP / Boreholes package? This question can elicit an uncharacteristic coyness in some GSHP salesmen, who will try to keep this information to themselves until a very late stage in your negotiations. Or, in the worst case, try to mislead you by understating the borehole depths required in order to make the project look cheaper and their sale easier.
If you have been given the required GSHP capacity in KW and the corresponding estimate of borehole depths required, check the calculation used. There are several obvious factors that influence the total cost. The size and capacity of the heat pump required is the most important.
A small 2-3 bedroomed house may require a heat pump capacity of 6 -10KW and around 100-200m of boreholes*, a larger residence with 4-8 bedrooms could need a 30-40KW pump and 500-900m of boreholes*. Other factors also play a significant role, the age of the house, quality of wall and roof insulation, suitability of distribution system, e.g. is there underfloor heating? Also, is the GSHP just required for heating, or also for hot water or even swimming pool heating?
Having said all of that and emphasizing that each case will be different, you must expect to pay typically, for the size ranges above, somewhere between £1400 and £2500 per installed KW of GSHP capacity(excluding VAT). This cost does not include any other equipment for distribution, e.g. underfloor heating, etc.
The cost per KW will be towards the high figure for smaller installations as the one off, transport and set up costs for the installer carry more weight.
As with many other things in life, the more installed KW capacity you buy, the cheaper the unit price becomes.
In terms of cost per square metre of heated area (assuming underfloor heating)
Both of the above graphics should be used only to arrive at a rough dimension for the project. Many factors can influence the cost upwards or downwards.
The total cost of your project is likely to be composed as below
(As an aside, this also shows that the drilling of the boreholes is very likely the single largest cost component in your project. Saving money by hiring the "cheapest" contractor can mean that costs will be saved, corners cut, where you will not see them. Faults in any part of your system above ground are relatively easy to correct, up to and including total replacement of the heat pump. Defects in the boreholes, leaks/kinks, drilled shallow, substandard grout, etc. will significantly detract from the overall performance of the system (and your hoped for savings!) and, as a rule, cannot be repaired.
* These figures can vary, mainly driven by two factors, 1) the efficiency of the heat pump, and 2) the thermal characteristics of the ground.