I remember there being a discussion on this (btw your question is not stupid) at the USO a few years ago because some of best umpires went missing due to the low rates. $500 a match I think was mentioned but not sure. That's for umpires.
Interesting. Do you happen to know the average amount of matches that an ump officiates annually? Seems like a low amount pet match, especially since they have to travel just like the players.