If you got less than $20k for your "internship" you got ripped off and they got their money's worth and then some.
I worked for a guy for nearly 4 months at a sub-par pay. That work turned into an opportunity that brings in six figures every year.
I understand where you're coming from, but sometimes, you have to give before you can get.
But, if they don't than I'm 100% with you. It's better to work on something worth doing for less money, then work on something you don't feel passionately about for more.