Unpaid job is an oxymoron. If they can't or won't pay you, it's either volunteer work, or a hobby. Why do charity work for a business that thinks it's going to be big; there's plenty of funding out there, let them find it and then they can pay you, and you can do work for them.
If you want to do an hour or a day or maybe even a week of work for free, ok. Maybe that's fun and interesting and you get something out of that, but after that if you're not getting anything, why would you keep doing it?