As of March 2015, WhatsApp is free on iOS and Android devices for the first year according to the app's site. A subscription fee is required after the first year. WhatsApp never automatically charges for a subscription fee.
Details of Whatsapp's subscription fees are available on its site in the FAQ section. Deleting and reinstalling WhatsApp does not reset the subscription. Messages can be sent and received freely with a subscription to WhatsApp, but data costs are still incurred at a normal rate. WhatsApp recommends using a Wi-Fi connection to send messages. Ensuring that roaming is turned off can minimize data costs.