In the for-profit world, people are hired on commission all the time. In other words, they are paid a percentage of what they bring in. In the nonprofit world, this is considered unethical. These resources explain why.
The Association of Fundraising Professionals (AFP) exists to foster the development and growth of fundraising professionals and the profession, to promote high ethical behavior in the fundraising profession and to preserve and enhance philanthropy and volunteerism. Read their Code of Ethical Principles and Standards.
In his article Motivation: Mission or Money? Author Walter Sczudlo explains how percentage-based compensation can encourage abuses, jeopardize the public’s trust and undermine the philanthropic values at the heart of the nonprofit sector.
These Blue Avocado articles In Search of Unicorns: Finding & Hiring Grantwriters, Part 1 & 2 offer excellent insight and advice on what you need to do to successfully and ethically work with a grantwriter.