(See the answer to "Can I award a bounty to an old answer?" in this [FAQ list][1])

Rewarding a "nice answer [which was not] sufficiently upvoted" is one of the intended use-cases of bounties.

The benefit for the answerer is two-fold: he gets the additional rep from the bounty, and he gets upvotes from the extra-visibility given to the question.

Another course of action, for users who want to reward an exemplary answer but do not have enough reputation for a bounty, is to draw visitors to the page. For example, you can feature the question in your blog, tweet about it, and so on. 



  [1]: https://meta.stackexchange.com/a/16067/266014