An idiom or expression (in American slang) that denotes an estimate of an amount or a numerical value for which no previously available accurate figures are there. It is an educated guess or estimation (or a roughly accurate approximation) within acceptable upper and lower levels. A ballpark figure might be the cost of a product or the value of an asset/ liability, etc. An entity attempting to determine losses associated with a project may use a ballpark figure for such projected losses.

A “ballpark figure/ estimate” implies an off-the-cuff guess or an approximation.

This expression comes from baseball denoting the baseball stadium, which was first known as a ballpark in the 1890s. The ball park (ballpark) has a set number of seats. On the day of a game, the commentator used to estimate the figure of people attending the game at the stadium. Hence, the expression “ballpark figure”, i.e., a rough estimate based on an indication.

Probably, it originated in close connection with the idiom “in the same ballpark” which indicates “approximately the same amount”.