Being underpaid is worse than not being paid at all. You know what you’re getting yourself into with the latter. You are a volunteer, so you can unvolunteer at will. You can prioritize other jobs above the non-paying “favor” job and expect that the person benefiting from your unpaid labor will, if they have any common sense, understand and you can decide just how much effort and time to expend because you’re the one holding the magnanimous cards. Not so with underpaying gigs. It’s all the work and the demands of a well-paid opportunity, but with a lot fewer zeros.
-- J. Maureen Henderson explains why working for less than you're worth is for chumps.