A LOT. Even in developing countries people paid ₹50 (or the equivalent of a $1). WhatsApp changes this model a bit by partnering with carriers. Unfortunately my memory is not clear on whether that was before or after the acquisition.

Edit: Note: WhatsApp publicly claimed that a billion users paying $1 each is enough for them. Also remember, a lot of the technical complexity, infrastructure requirement and data storage requirements went away in their model of being just a conduit. They had no interest in reading, parsing, mining, storing or monetizing the messages. This let them run it with a lean crew.