One only has to read the whole story, which by design nobody watching a 30 second spot on the news is getting, to fully understand why replacing food storage units on this one of a kind A/P cost so much,, and working for Boeing I can understand why all the cost from enginnering to instaltion could make it an expensive project. Much more so I. Bet if this was a one of a kind AC unit and all its ducting. The picture the fake news wants people to hace is far different from reality.
Build just one of anything to install on an airplane, which has to perform with extremely high reliability and very low probability (approaching zero) of creating other problems. Yep, it's going to be expensive.
When was the last upgrade (1990), and will this one even use the same refrigerant? There is a lot to consider, even if this was going onto some other jet, but on the one which transports the POTUS, it has to work right first time, every time, and remain reliable, and there are other unique systems on board the aircraft to consider as well. They added in the cost of testing the new components for reliability and safety, and installation won't be simple, either, which translates to expensive.