In major events, are tennis balls reused? For example, in the Australian Open, the ball boy/girl will give a player three balls; the player will reject one or two, and keep the rest. What happens to the “rejected” balls? They have been touched with the player’s racket or hand (gently tapped/tossed out of their way). Can they be reused?
Does a new ball come into play at the end of each service? What happens the the “used” ones? Sold to fans as souvenirs (“Nadal hit the net with this ball”)?
At the start of the match 6 balls are used, and then the balls are changed to a new set after the first 7 games. After this the balls are changed every 9 games.
So ball changes occur after the 7th, 16th, 25th, 34th, 43rd, 52nd games etc…
The reason for a quicker change near the beginning of the match is because the players warm up with a set of balls and use the same set for the first 7 games.
Not sure what they do with the balls after they are withdrawn from the match. They can’t be used for match play after that, but they could just be practice balls.
They might sell them? I have a couple from the US Open years ago(I bought them from an estate sale). People collect them and use them to get autographs.
They definitely sell them at the 4 major tournaments. My ex has a set of balls that she bought at Wimbledon. IIRC, they claimed to have been used in an actual match, but nothing beyond that - so they could have been used in round 1 between a pair of low ranked players.
Wimbledon have a special ‘used balls’ kiosk during the tournament. Proceeds go to various charities and is managed by the Wimbledon Foundation.
Slight derail - seen a trend (only in recent years, to be sure) for spectators to throw the ball back into the court again.
Meh.
Keep the damn thing. Tournament organizers can afford such loss leaders.
Heh - major ball derail - nice to see Shapo hasn’t been cranking the ball into any officiator’s face lately. 
I don’t think money is the problem. Since balls are supposed to remain in play for a particular number of games, some players may play better with newer (or, for that matter, older) balls, so replacing a ball early may give one player an advantage. I remember a few years ago where a major player held up a match because a ball was hit into the crowd and wasn’t being returned.