Great question. I'm the founder of https://www.GetFoundMarketing.org and we've done this with select companies/startups.

As it turns out, many of the answers here are on point. And much of the time, a successful marketing group will need some "core stipend", to make it fair for both parties.

With that being said, I think you would need to have an indepth conversation with the marketing company you'd like to propose to get a better idea of your product/service that you are offering to the masses.

Much of the time, we make decisions based on the business climate of the potential product or service. We analyze the value as well, because as superstar marketers, we realize that you still have to have a truly great product or service in order for us to make your message ring loud and clear and make it profitable for both parties.

If you want to discuss, we are an open ear.