Why do you think that hosted cochroachdb would be cheaper then hosted spanner? Google has been optimizing spanner performance for years so I would expect that it will be cheaper to run for quite a while. Of course the markup can be different but I wouldn't expect it to make a huge difference.
Maybe training? Spanner's lack of UPDATE/INSERT/DELETE requires a team that's trained pretty specifically on how it works.
Frugal, froogle was the old name for google shopping.