Hate to show my age but .... in the beginning (of FF programs) the minimum was 1,000 at least on Delta, Eastern and Pan AM (Republic Airlines used a a passbook like they used in banks years ago and it was stamped by the GA when you boarded), the airlines I used the most. The really best promotion they ever ran was "Triple miles" l flew MCO/MIA every 2 weeks back then and racked up 8,000 miles on a $79 R/T ticket (6,000 from the airline, 1,000 for a National and 1,000 from Marriott). I mostly flew Eastern in those days and was what they called an "Executive Traveler" which was the precursor of the "precious metal ranking used today.