The Kik Messenger app is free to use and is offered as a free download, as of 2015. However, users may incur data charges from their cell or wireless carriers.
The best way to avoid being charged by a mobile carrier is to use the app on a Wi-Fi network. Using the app while away on a mobile network will likely result in fees being charged. Always make sure Wi-Fi is enabled on the phone and check to determine how much data is included in the wireless plan before using the app. The app, along with its content from sources such as YouTube, may result in charges.