As highlighted in the fees section, the center referee gets $70 and each assistant referee gets $50. That’s $190 in total, split between the two teams at $85 a piece.
If you only have two referees when the game starts, you are not obligated to provide for the late/non-existent third referee. If the referee arrives late, you can take care of that at halftime.
Use your discretion – they do work hard, and covering the whole field is a lot of work. Especially if you are short-handed.