The app itself is a free download, but their are many different plan which customers purchase, such as the $29.99 all you want Worldwide calling plan all the way down to the most popular Pay & Go which allows a customer to prepay for as little as $1 dollar on the card and they are only charged when they actually make a call and use the service.
Most people add 15 to 25 at a time on the prepaid card and replenish the balance as needed based on their use.