It may depend on the type of promotion you need, and it will definitely depend on your budget. As you know, a PR agency will want money. Do you want the person placed in the media (interviews with industry writers, etc.) or promotion for various gigs? Are you wanting a new image? All this things take time and money to execute.
If this is a new artist, consider looking for a hungry PR person just starting out. You can sometimes get PR students or even an entire class to take on a project (if there's a local university) for part of their graduation requirements. I think some PR majors would live to have that in their portfolio. Maybe you can have a win-win situation.