How much to pay for another company's marketing?

Hi - my first post!



I am a sole trader, and my business is personal coaching/personal development and healing courses.

I work with a small college who recommend me and from whom I get students for my courses - probably about 12-15 students per year.

My question is how much, what percentage makes sense to pay the college for their marketing? I've heard that 15-20% of profit is about the going rate.

Any ideas?

Thanks in advance.

:)