If you can get the job done with volunteers, do so. If, however, you are pressed for time or cannot find someone to handle a specific (and necessary) task, go ahead and hire someone, requesting some consideration because you are a nonprofit group or simply a group of people staging a fundraiser.
You should not work out a deal offering to pay someone based on the success of the event or fundraising campaign. Professionals are doing a job for you — they should be paid for the service you hired them for, regardless of factors out of their control (such as a low attendance rate).
Most often, a professional — whether it's a clown to entertain children or a lawyer to handle a major lawsuit — will be paid through your organization's treasury, regardless of the success of the fundraiser. When retaining professionals who are interested in your cause or mission, you will likely find they will be more accommodating with their fees.
The bottom line: You need to be able to justify paying someone for services. Can this person bring you closer to your goal? Do the policies, bylaws, guidelines, and, most importantly, the members of the organization support the hiring of a professional? Whatever your answer — and your final decision — use it to set an ongoing precedent for the future.