Work for equity?

I have a fulltime job but I do some contractor (programming) work on the side to earn extra cash. My client would like me to work for equity. I like his startup and I think it has some potential. The contract talks about a 3% equity interest vested over 2 years (50% the 1st year). If either party terminate the contract the 1st year, I don't get anything, if it's during the 2nd year, I get whatever has been vested. At this time I don't know the current valuation of the company. But 3% of a startup seems low to me, especially when you include the 1st year risks. Do you have some advice or previous experience? I don't have experience with this kind of thing, would you consider that as working for free or even some kind of lottery? Can I cash the equity at some point?