You were willing to work for nothing because of what you might learn. For one thine, you didn't know that you had skills that were worth money. Now you see that you do. You probably learned a lot of other things too. I also think that an internship should always be a two way street, with each party doing something to help the other. I would certainly expect the person in your position to make a contribution. The other guys are spending time on you, and taking a little bit of a risk.

You already wrote off the summer and the paycheck. Do what you can and see how it plays out. You could be pleasantly surprised. The probability of that certainly goes up when you do something of value.