Whether to "buy" miles or not depends on how many AA miles one has already accumulated as well. If I have zero AA miles, I would buy one mile at 1.5 cents with no problem, but with existing 200K miles I would be hesitating to pay 1.5 cents.

The same thing applies to the decision of whether to work or not. If one is wealthy enough, he would not work 1 hour for 20 bucks each, but if he is cold and hungry, he would work even at 5 bucks.