ballpark figure - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
An acceptable, roughly accurate approximation, as in I know you can't tell me the exact cost; just give me a ballpark figure. This term alludes to a baseball field, which is always an enclosed space. The expression is basically an extension of the somewhat earlier in the ballpark, meaning within a reasonable range, and out of the ballpark, beyond a reasonable range. [Slang; late 1960s]