The entire user database and connection system. The ability to add friends, build your profile, etc. Most static data would be included. The ability to add photo albums would cost more and so would the live chat because those are less common chunks of functionality.
I don't want to ruin this here but for a good coder, the register, log in and friendships pages are bullshit. Also the chat that you want would be a easy task for a jQuery programmer. I'd pay more attention at the design, a good custom template can have a pretty high price.
Lumpy, are you saying here that the register, the login and the friendship pages are easy tasks or hard ones ?
Everything is hard one, design should be simple, but functional so can't be done without some planning. Register, login etc all need database design. My estimate if we are doing this custom will be more then 2000 euros. If you can find someone to implement this in custom WP theme with BuddyPress maybe less.
If you want voice chat, I think (only a gut instinct) you'll need a dedicated server, which can cost $50-$100 a month. You can create voice chats with flash, but you'd need a text fallback for people without a microphone. One can be doing voice while the other has to use text. I'm pretty sure there's a flash component out there already that does this for you. You'd be wise to find and leverage that, rather than have something created from scratch that might not scale to large user numbers properly. As for cost of creating the web app, I'd charge; * user database + secure non-https login system : $150 * friends list in database and friend-request system : $100 * graphical layout and code for the site : $150 * plugging in of pre-made flash chat : $100 But you gotta know, facebook has a lot of clout. It'll be hard to squeeze into that market and keep your site populated with active users I think.