... as you claim you are. Never work for free unless you are somehow benefitting from it; either financially, from a unique experience, or to take part in something you believe in (charity work, an open source project, etc.). It is a known fact that people associate your quality and value with the rate you demand. Granted, some people are simply delusional with the thought that they are better than what they are. However, in the end, if you work for free then that must be what you're worth.
If your work product is good enough that your boss wants you on it, then he should be willing to pay for it. I would simply send him a polite email explaining your change of heart with the reasons behind it. As long as you are willing to stay onboard, offer to continue for your normal rate... or even a slightly higher rate if you are working without benefits or a guarantee that it will last. Make sure to copy your personal email account so that you have some type of documentation showing that you didn't simply leave him high and dry.