Some agencies and consultants, if they are really good at what they do, can demand revenue sharing. Meaning that they provide a service (SEO, for example), and then get a piece of the revenue - given that they hit some threshold.
In that case, the sky is the limit. I met one consultant that netted nearly $10m a year doing that - he had specialized on SEO in a cash-rich field, and got some pretty big clients. In his words, hourly billing is for suckers - there's a natural limit for how many hours there are, and the only way you get more revenue that way is by increasing the number of employees. Apparently when going from hourly billing / fees, to revenue sharing, he jumped 10x-100x in annual revenue.
Of course, most can't just demand that - and many clients will refuse. But some will, if you're good enough.
In that case, the sky is the limit. I met one consultant that netted nearly $10m a year doing that - he had specialized on SEO in a cash-rich field, and got some pretty big clients. In his words, hourly billing is for suckers - there's a natural limit for how many hours there are, and the only way you get more revenue that way is by increasing the number of employees. Apparently when going from hourly billing / fees, to revenue sharing, he jumped 10x-100x in annual revenue.
Of course, most can't just demand that - and many clients will refuse. But some will, if you're good enough.