Over here in northern Europe, for a typical single-family house, replacing an oil-fired boiler with an air-water heat pump (that is, the heat pump uses outside

Over here in northern Europe, for a typical single-family house, replacing an oil-fired boiler with an air-water heat pump (that is, the heat pump uses outside air to heat water that can be run through the existing radiators) costs around 10k-15kEUR, replacing the boiler with a ground-source heat pump (incl. borehole drilling, the whole shebang) is around 15-25kEUR. So the price difference is not huge. In southern parts of the country, and for older houses with less expected life left in them, air-source pumps have proved more popular, whereas further up north and for newer buildings ground-source pumps are more often the choice.