It should be free. A beta is a beta - it's not finished. The reasons for your beta period is for YOUR sake. You are the one who wants to test your product, give

It should be free. A beta is a beta - it's not finished. The reasons for your beta period is for YOUR sake. You are the one who wants to test your product, give it some marketing and get feedback before launching a final version. The users in that case are helping you achieve your goal - which means that they should get to try out that service for free.