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]

(orig. US) used in combs. below, all meaning approximately accurate.

In compounds

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

(orig. US) a round figure for general estimation, assessment.

[US]Wall Street Journal 7 June 4: I gave them a guess of somewhere around $1.5 billion [...] I think they accepted it as a guess. I thought it was a ball-park figure.
[US]Time 3 Nov. 50: His uniform’s cost? ‘Two thousand,’ said Elton. ‘But that’s just a ballpark figure.’.
[US]D. Burke Street Talk 2 169: I can only give you a ball park figure.
[US]F.X. Toole Rope Burns 17: Ballpark, I get first cut of the purse, 2 percent.
[US]T. Dorsey Stingray Shuffle 44: Just give me a ballpark of how long the wait is!
[US]R. Price Lush Life 294: CP... CP one? CP five? Ballpark...

In phrases

in the (same) ball-park

(orig. US) approximately accurate.

[US]Current Sl. II:3 1: In the ball park, adj. Approximate but not exact.
[US]S. King Stand (1990) 796: We can make a pretty good guess that figure’s in the ballpark.
[US]Tarantino & Avery Pulp Fiction [film script] 13: Is it as bad as eatin’ her out — no, but you’re in the same fuckin’ ballpark.