$74 a game (at least, out here in California), at the high school level. $120 at the junior college level. Drops to about $40-50 at the PONY or Little League level (PONY hires umpires, Little League has a mix of pay and volunteer–they prefer volunteer.)
Out here, for high school and PONY you pay the assigner $4 a game to hook you up, but that is minor compared to what you can make. When DH was unemployed for a year, baseball season saved us. He brought in about $800 a month for the 4 months of regular season.