One is expensive, but most of the costs is setup/teardown so you just produce a bunch (perhaps 1000) for the warehouse when supply runs low. That is how companies that keep their molds function, but you pay for them to store the mold and have a warehouse with all the supply in, so spare parts are always a lot more expensive after full production has stopped.

Don't confuse spare parts with maintenance parts. If there are wear parts (filters, bearings...) that are expected to be replaced regularly there is a lot more turn over of the parts, and so everything is planned ahead around keeping the right amount of production going. These parts tend to be more industry standard parts bought off the shelf from a supplier that has other customers for the same part so your part order might go to the supplier who just puts a different label on the standard part and ships it to you.