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It really isn't very expensive. Installation is mostly connecting the inside unit with the outside unit through an 8-10mm pipe (1/3"). The work is about 2-3 hours and material costs are pretty insignificant. The HX units are pretty costly, though. About $2K for a decent one.

Where I live, Norway, a lot of houses have gotten these retrofitted.

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Oh that's interesting. What I've heard is that changing the heating system from radiators that use water heated by gas (as is common in German gas heating systems) to a heat pump is difficult because heat pumps don't give you such high temperatures. So you'd need a lot more surface area to heat adequately with the lower temperatures provided by your heat pump, i.e. you'd need to switch to underfloor heating, which means ripping open all your floors. Glad to hear that there are apparently simpler ways. We need to switch almost all buildings over to heat pumps over the next two or three decades after all.

AFAIK it's not ideal but possible to use existing radiators.

My parents recently added an air heatpump to their existing (wood) heating system without changing radiators. They Installed a new boiler (water tank, not sure about the translation) where the top half is heated by the heat pump and they can optionally heat the whole tank using the old wood heater.

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