A little over two years ago American introduced multiple levels of prices for these higher-priced awards. Occasionally these cost 20% fewer miles, most of the time they cost a lot more miles.
The price did not vary by flight (how sold out it was, or how likely to sell out) but by date. So some days were more expensive than others. You could still book the last seat available on an almost sold out flight for the lowest extra miles cost if it fell on a lowest price date.
THIS NO LONGER HOLDS TRUE at least for New York JFK – Los Angeles and New York JFK – San Francisco. WITHOUT NOTICE American changed how they manage awards on these flights on Monday August 15.