Depends completely on the traffic. How many people that actually use the website, that's how you would determine it. I know for sure that it would not be cheap; you need to send a lot of data around. My suggestion is to first have a mini beta testing period.
I was afraid someone might say that. It's hard to determine the amount of people you are going to get to your site. Any estimations with 100-1000 people a month. I can't see me getting more than that its not like the site is original just a side project I would like to do.
That side project may be costly. You would have to spend a lot of secure servers to make sure your users' chats are secured, plus the load will be pretty big. I have never done anything with a userbase of 1000 active people, especially not on webcam chat sites... so I wouldn't really know.