It really depends on what is being designed.

I'd be really careful about using the term 'consultant' as it implies a certain, very high level of expertise in the industry.

And usually it is very very specialized. Just because you may know about business printing doesn't mean your knowledge would carry over into the exhibit industry (generally it doesn't).

A Graphic Design Consultant? Hmmm.... Unless you have a hefty marketing analysis group to back you up, I'd avoid it all together. Your price would be based on how right you are and how much market share you can garner for your client.

I've worked with consulting firms for the design industry but only in the exhibit-related areas so I can't help you out much with 'normal' graphic design. We actually do consults here quite often too, suggesting methods or alternate methods to meet an end result. Fees are usually based on project complexity.

Payment should always be involved for ideas. You have to be very careful not to confuse consulting with spec work. Don't offer ideas in hopes of landing a project.