An appraisal of the stake, not a thesis. The sentence asserts nothing about its subject — it re-weighs what has been under discussion. The word ἀγών means both a contest and a lawsuit: what looked like a classroom analysis is called a fight, and one with its price named.

What exactly is opposed. οὐ περὶ τοῦ τυχόντος — "not about whatever comes to hand" (τυχών, "what has happened along," hence "ordinary, trifling"); ἀλλὰ περὶ τοῦ μαίνεσθαι ἢ μή — "but about being mad or not." The formula is built on a gap of scale: on one side a trifle, on the other one's reason. No intermediate degrees are offered: either — or.

Why μαίνεσθαι. Madness in the Stoics is neither a metaphor nor abuse but a technical limit: the non-sage, living by false judgements, is literally out of his mind. The same word describes at 11.33 the man who looks for his dead child (μαινομένου), and at 11.18 the demand that bad men not go wrong (μανικόν). Three cases in a row, and in all of them madness is the mismatch of a demand with the constitution of things.

Where the sentence stands. Directly after 11.37, with its enumerated departments of the work: an art of assent, a guard set over the impulses, abstention from craving. The passage reads as a reply to an objection — "is it worth all this trouble?": the list looked like a schoolroom method, and now its subject is named. The governing discipline is assent, the sustaining one action.