If you put a bounty on a question and the question fails to get new answers, the points attached to the bounty expire unused. It makes sense that the bounty points do not get "refunded" to you, but I think it would be a good idea to let users get "store credit" for them.
For example, if you put a 50-point bounty on a question, fail to get new answers eligible for a 50% auto-assignment, and let the bounty expire, you should be allowed to put a new bounty of higher-value on the same question, "paying" with a mix of the expired and new reputation points. You might put a 100-point bounty for additional 50 points, or a 250-point bounty for additional 200 points, et cetera; the other 50 points would come from the "store credit" based on the expired bounty.
The expired 50 points would remain attached to the question on which they were originally placed; they would not be transferable for use to set bounties on other questions.