What does a referee make at the US Open for tennis?

Well, if by 'referee' you don't really mean 'line umpire' or 'chair umpire', then I would say around 10k for the tournament depending on the level (there are deputy referees and then the head/tournament referee). line umpires make in the mid-100's range per day. For chair umpires, it depends on what level they are.