Little League provides pretty good training (at least out here in our area) and the rules in high school and PONY are not much different (substitution rules in high school, and the age where you can lead off for PONY). Even if you spent a season volunteering (for free) just to get the experience, you would be pretty far ahead.
$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.