The goal is to provide full privacy for users so it would be impossible to understand who is the author of content, his location, preferences or wether two users have communicated. How much would that cost and who is capable of doing this kind of work?
It's a bit difficult to give an estimate with the information provided.
I'm not sure if your question is related to adding privacy to an existing server code or you need the whole server side done. The later is significantly more cost and has an unknown scope based on only the info given.
You might also want to look at platforms like Parse and see if you could get all you need done there vs a custom server side system.
Happy to chat more on a call if you want to provide more info or have more detailed questions.