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According to How does the bounty system work?, the minimum spend on a bounty is 100 reputation if you already have an answer on that question. I get that, and it is explained in waffles' answer to Clever bounty reputation hack.
Is this by status-bydesign to prevent abuse? What if I self-delete a poorly-received answer (or a 0 score answer) and want to add a bounty to a question I either attempted to answer or answered incorrectly?
The higher requirement is to prevent you from driving traffic (and thus potential votes) to your own answer. The idea is that a bounty should be a real expenditure, not something you can expect to make a profit on.
If you deleted an answer, you can undelete it. So without this limit, you could delete a good answer, post a 50-point bounty, undelete, and then collect upvotes. The SE engine can't tell whether you deleted your answer because you're withdrawing it or because you're trying to game the bounty system, so it just requires the higher value if you have any answer there.
(Perhaps the bounty system could be improved to only raise the limit if the answer was deleted recently or has a score over some threshold, but somebody would need to make a case to justify the extra complexity.)
The SE engine can't tell whether you deleted your answer because you're withdrawing it or because you're trying to game the bounty system maybe there could be enhancements put in place to detect this kind of fraudulent behavior.