I thought that, in general 50 is the minimum level for a bounty (as an example, this question currently has an open bounty for 50 rep: Associating chords with scales )
But when I added a bounty to my question, the minimum rep I could offer was 100 (I think the levels were 100, 200, 300, 500 though I'm not 100% certain).
I just tried adding another bounty and the levels were all in 50 rep intervals. That was definitely not what I saw previously, bounty reward levels were all multiples of 100 so it wasn't just the 50 which was missing.
Why was this the case?