I only buy bonus fodder when it is about 500 treasure more than the non bonus fodder. This is because I buy fodder early in the day to get good pricing but can't train until later in the day. And sometimes something comes up and I can't train at all. And paying more for fodder that could become non bonus fodder is then a waste.

Of course I'm not typical, and I don't buy fodder as much these days anyway, as it is too expensive to be worth it to me. (Why spend 7-7.5k on fodder to get just a couple of thousand profit per dragon after having to deal with the frustration of dodging enemies when I can get 75k in 15-20 minutes by playing glimmer and gloom?)