Peter,
This is a touchy subject, but the basic premise is:
It is all negotiable. What are they offering? What are the guarantees, if any? Do you feel like you will get your monies worth at the price they are asking?
I usually look at all of the above and then decide based on these items plus gut and a “you get what you pay for” philosophy. I have paid as much as 15% of gross revenue generated, a monthly retainer with minimum guarantees and some commission, say 3% and then retainer only. Get references and see how these guys paid the consultants. Also, see if these clients felt they got their monies worth. That is the most critical aspect.
Good Luck,
Tim Harvey