According to MLB, between five and six dozen are used per game (60-70 balls).
Have to have 90 on handThe home team has to have 90 new baseballs on hand for each game. That doesn't mean they'll use them all, but they must have them, just in case.
Average life of ball is 6 pitchesThis is an interesting question. I am not sure if a true average can be reached due to climate and other issues and yet here is what I found on the web through yahoo. According to Ensley, "every major league game begins with six dozen balls." He goes on to explain that the average life of a ball is six pitches. Since most major league games average between 250 and 300 pitches, that would put the ball count at about 40 or 50 balls used per game.
About 72 ballsAccording to the MLB: "between five and six dozen baseballs are used during each baseball game, as many as 72 balls."
Each ball costs about $3-dollars, so that works out to $216-dollars worth of balls for each baseball game.