I would assume that they're paying out of pocket for the STC as well as the costs of parts and installation (which would make that cost virtually nothing if they use their own mechanics during routine overnight checks) instead of waiting for UA to approve a 3rd party. Since it's now what UA wants, they'll get their money back eventually.

I would assume that since the Mesa birds are owned by UA, that UA will have to approve a contract MRO to install it. Republic should be able to buy the STC and do it themselves, though, since they own their planes.

The E-175s should get done fairly quickly since there's not that many to do right now. The CR7's might take a while depending on how many MX bases get involved.