YaMei, The expression "in the ballpark" comes from the American game of baseball. A ballpark is the place where the game of baseball is played. Being that you are from Taiwan, I imagine that you know what 'baseball' is. If a baseball goes "out of the ballpark" for any reason, the game stops until it is retrieved or another ball is put into play. Sometimes it is a good thing, when a player hits a homerun, for example. The game cannot continue unless the ball is in the ballpark. In order for the players to do their work they must have a ball to play with. Figuratively the expression has come to mean that if "something is in the ballpark" it is reasonable, or workable, for the purpose at hand. You can play your game (accomplish your goal) with it. "The Delphi technique of brainstorming can often produce answers suprisingly close to reality, but it may also produce one not even "in the ball park". OED Megill in 'Risk Analysis' 1977 A number is "in the ballpark" if it is an approximate or reasonable amount for a particular purpose. In your sentence "30,000" is in the ballpark. It is within the range of the possible; it is approximately the amount. If "30,000" was not an acceptable number you could say the amount was "not in the ballpark" meaning --not a reasonable amount for the purpose at hand.

"in the ballpark"here means "about"or"nearly",but I don't know when people use it.

Never liked this friggin baseball. Game for some dumb people who have nothin else to care about.