a ballpark figure
An approximate number; an estimate. Do you have a ballpark figure for the cost of the renovations? That's just a ballpark figure—they don't know exactly how many people will be attending the event.
a ballpark figureor
a ballpark estimate
A ballpark figure or a ballpark estimate is an approximate figure or quantity. Note: A ballpark is a park or stadium where baseball is played. But what are we talking about here — a few thousand, millions, two bucks? Give me a ballpark figure. I think just in a ballpark estimate — about 60-40. Sixty would support, 40 percent would be opposed.