The school district sets the stipend for each individual sport. For instance, every football coach in Greenville County gets the same amount from the District. Boys soccer will get a different amount than football and girls soccer will get a different amount than boys soccer.
Now, stipends differ from District to District (Greenville to Charleston, Lexington 1 to Lexington 2, etc).
Also, some Booster Clubs may be willing to throw in extra money to suplement what the District supplies.