Okay, keep in mind that this post is coming from a girl who hasn't
started camming yet, so do take my words here with a pinch of salt. However, I am doing as much research as I can before I begin, (I have ID to sort out), and judging by the blogs, FAQ's, sites, etc that I've found, there really seems to be no way of calculating an average or estimate. It all seems to be dependent on a combination of things: Which site the girl is camming on (being as they all have different rates), how many hours she is working, how much she charges, and how she acts in free chat to rope guys into buying private chats.
I've been looking at girls in the free chats on Streamate lately (which may be the first cam site I join), and I notice that some have scheduled chats DAILY (or nightly), so they are literally camming as a full-time job. However, some girls seem to take ages to get a guy into private chat, wheras some seem to constantly get guys into private chat. It depends on how they act, how they engage with their viewers, and how confidently they come across. There are some girls who say nothing, have no mic, and look very, very uneasy. They still get snapped up, albeit at a much slower pace than girls who really converse with their audience.
So essentially, you could look at one girl, notice she is going camming for a full time job, but she still may not earn nearly as much as the others. Taking these factors into account, it strikes me that if you're wanting big bucks, you need to put in the effort. It seems you can make your own average, setting your own definition: by making goals for yourself to work towards - working out how much you want, and working out what it is that you do on cam which GETS you what you want (guys going private with you / buying your shows).
If you want any links to the blogs, FAQ's, etc, that I've been looking at, feel free to pm me and I'll send you all the links. I've been finding them very helpful.
Best of luck!