Green’s Dictionary of Slang

ball-park n.

[SE ball-park, a baseball stadium, i.e. the rough estimate of the number of fans watching the game]

[1950s+] (orig. US) used in combs. below, all meaning approximately accurate.

In compounds

ball-park figure (n.) (also ball-park)

[1950s+] (orig. US) a round figure for general estimation, assessment.

In phrases

in the (same) ball-park

[1950s+] (orig. US) approximately accurate.