Ballpark Figure (1957)

“Pentagon language continues to produce new bafflers. One of them is ‘a ballpark figure,’ meaning a very rough estimate which doesn’t do much more than indicate that a given program is going to cost someone an awful lot of money.” The Des Moines Register, June 24, 1957, p16 

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):

1963. “As business heads like to say, these are good ballpark figures” (in reference to the costs for art as a percentage of the cost of an office or public building) (Los Angeles Times, July 4