This phrase is related to another popular phrase, 'in the ballpark', which means 'close but not
yet there'. The idea is that you can be in the ballpark (a place where
baseball games are played) but not yet in the right seat (the seat you
have a ticket for). The seat is a metaphor for the right idea or place,
and the ballpark is a metaphor for an approximation of that idea or place.
A 'ballpark figure' is a number that is 'in the ballpark' (close to the
true number) but not quite in the right seat (the true number).