The actual cost for building a service like Whatsapp is mostly dependant on the infrastructure to put in place.
The price for the application (iOS client) is then only a small part of the total cost.
Without knowing the infrastructure choice you made, it is quite complicate to estimate the work load for the iOS client.
The more complex part of such a project would be to manage the message status in real-time, connect to various APIs to enable friend invites to grow the network easily or integrate with their current iOS contact book.