I do not believe stage managers are paid extra for making a pre-show announcement.
Yes, ushers are paid an extra dollar or two if they have to stuff anything in Playbills.
An announcement about an understudy is generally avoided if possible for two reasons:: 1. There will always be a collect groan of disappointment from the audience. 2. The curtain is about to go up, but many people will start flipping through Playbills trying to find the under study's headshot and bio as well as the headshot of who they are on for. Neither of those things make for a pleasant curtain rise. Better to allow the audience to open the program and discover it on their own as well as to give the audience something in print to refer to later alongside their printed program.
("Back in my day" it was quite uncommon for tours to do inserts... It was near always lobby board and announcement. I generally always carry a pen on my person, so I have numerous tour programs where I noted understudy performances when they were on...)