For the benefits of the Android community, Google Play has rolled out a free trial period (minimum 7 days) for in-app subscriptions so that users can better decide weather the games or apps are applicable to their needs. Though it is free during this period of time, users are still required to enter that payment information. However, if you never cancel your subscription before the free trial ends, you will then be billed for the normal fee of the subscription.
Source : Android Developers