1
general::
Phrase(s): a ballpark figure
Fig. an estimate; an off-the-cuff guess. • I don’t need an exact number. A ballpark figure will do.
McGrawhill's American Idioms And Phrasal Verbs
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general::
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]
American Heritage Idioms