Can the bounty rules be changed so that users can offer bounties lower than fifty points? I think that Ask Ubuntu would benefit from allowing users to offer five-point, ten-point, and fifteen-point bounties. Some users may want to offer bounties, but they do not want to give up so many reputation points (like me for this question What are the steps to modify a filesystem (say XFS)?). If users could offer smaller amounts, perhaps more bounties will be made.
Can the bounty rules be changed so that users can offer bounties lower than fifty points?
You can make that amount in upvotes easily. If bounties are so low, they diminish in value, when you give a bounty you're effectively saying "I really want eyes on this question, so much so I've given X of my hard earned reputation". If you're uncomfortable giving the minimum 50 rep, especially if a question is difficult, a 5 or 10 rep incentive is tantamount to just one or two upvotes. You have to just weigh the cost vs desire.
That being said, there are a lot of things you can do other than giving a bounty to bring attention to your question. Editing it to provide steps you've done (not in this case, since it's not your question), etc can help bring attention to a question.