Ideally you want to get paid, you'll have plenty of time to be broke while you start school. But if it is a one in a lifetime opportunity with possibility of publication, it's worth volunteering.What it really comes down to is do you need the money or do you have other resources(living with parents, savings...)? Maybe you could vounteer and get a part time job? And remember the summer before med school is not your last opportunity to do research so have fun also!