When working there is really only one rule:
Work for free because you love the cause that much. Work for free because you chose to; not wanting the laws of currency to contaminate the art and soul of the work.
Or work for full price, getting what you’re worth and not a penny less. You worked hard to acquire the wonderful skills you have today, might as well treat them as such and attain your rightful fee.
But never work for cheap. For this taints the love of working for free, and in the process you’re disrespecting yourself and the skills you have attained. Working for cheap scorns not just you, but the work as well.