In my case, my clients are companies and they have a price defined by their employees number (X dollars to pay if the company has more than Y employees, and so on…).
Why I do not establish a plan per employee using the product? Because the product mainly helps the company’s employee to save money, not the company itself (even if, of course, somehow the company will benefit from that)… but I (really) need the company to promote it to its employees (not be afraid of doing so because the company would pay more if more users use it)…
(And the more employees the company have, the more support we need to dedicate to them.)
At least, with a fixed (low) price (established on their company category -> more than X employees), there is “no conflict of interest” and they can promote the product to make their (little) investment worth it!