Second, I think there is a mistake in the last sentence. I think it should be "Conquer, but conquer AT your cost."
To me this now means, that the owner is telling the donkey (ass), that he (the donkey) can do what he wants when he is not working for the owner, but while he is working he must do what he is told to do.
We typically say "do it AT your cost" to mean, do it when it does not affect me!
I am not a teacher -- just a native
Student or Learner