Husband: Lily, you play entirely too many video games. How much have you spent on games this year?
Wife: Oh, something in the ballpark of $500. That’s not so much!

The ballpark is a place to play baseball. You need a lot of space to play baseball, so it’s a wide open area. When a number is approximate, we can imagine it being close to the actual number, or in the same general place. If the real number were a point inside a ballpark, our estimate might not be the same number but it would be located in the same ballpark. You can also use “in the area” or “in the neighborhood” in the same way.