The heater pump when originally manufacturer costs £20 because it is done in lots of (tens of?) thousands and assembled in bulk by cheap labour, and shipped in a large container that costs not-a-lot of to move over an ocean.

The replacement part is stored by (potentially) a third-party company that ordered it for £30, by an employee who is paid £40,000 per year, in a warehouse that costs >£100 per square metre to rent (potentially for years), and when requested then has to be delivered by someone paid £20/hour, to be installed by someone paid >£50/hour (with a three hour minimum). And don't forget the VAT.