The help center on bounties currently only states
To avoid overly promotional bounties, if you are offering a bounty on a question that you have already posted an answer to, your minimum spend is 100 reputation (not 50).
This lacks the information that the minimum amount for a bounty on the same question by the same user doubles everytime, i.e. it's 100 for the second bouty, 200 for the third bounty, 400 for the fourth and so on (well, and so on is relative, since the maximum of 500 is still place, and I haven't yet tested whether a fifth bounty will have to be 500 or simply cannot be done. But I digress...). So the text should be extended into something like
To avoid overly promotional bounties, if you are offering a bounty on a question that you have already posted an answer to, your minimum spend is 100 reputation (not 50). Also, if you are offering a bounty again on the same question, the minimum spend doubles each time.