Brennan has some really good points here.
(Disclosure: operated my own PR/Marketing biz for 20 years.)
However..."The marketplace is loaded with consultants and marketing specialists ...Sometimes the appeal is due to the fact that they know a lot about marketing and you don’t."

Business owners and operators: what they know about marketing is secondary.  You'd EXPECT that they would. The question is, how eager are they to learn about your business? Another one: how interested are they in your business that they ask the questions that need asking? If you don't know answers, that's okay...before you spend a cent.  Sometimes a marketing plan is step one--and that means knowing your business and finding answers WITH your consultant, if you can...and sometimes you can't.  But know that ahead of time.