Well, the biggest cost here wouldn't be in the app, but in the server infrastructure. As usual with connected apps, people often forget they have to be connected to servers. The app itself is just a view to what the servers provide, so it's not the major cost. 20K $ to 100K $ should be enough.
On server stuff, I expect it to be a lot more, particularly if you aim for a wide audience (several million $).
On a Whatsapp clone, I'd consider:
- solutions for performing calls through servers, not that easy. Look at what Twilio provides. WebRTC is an upcoming protocol that will ease that (web & mobile).
- solutions for API, IM and user accounts management. Look at StackMob or Parse which are MBaaS (Mobile Backend as a Service).
Hope it helps!