At least a dozen times this season I've personally witnessed small kids talking about what they could buy with the money they would get from selling a foul ball that someone gave to them (out of guilt or public pressure) while their parents asked if they wouldn't rather keep it for the memories.
However, my home team is the Dodgers, so the experience is probably different in other markets where the local team sucks and nobody cares about the players (like the Anaheim Angels, where fans are more likely to throw the ball back onto the field than keep them).