There was a data leak 2-3 years ago from one of the cam sites and they accidentally posted a spreadsheet with info about their cam models.  The top 6 or so were estimated to be making over $100,000 per year. That is, the data was in terms of "coins" or "tokens" or "credits" or whatever guys pay with.  Guys pay around $X per credit and models were assumed to be getting Y% of the value and that's how the $100,000+ figures were estimated.  
.
The top models were porn stars or recently retired porn stars.  Near the top was Sammie Rhodes.
.
Earnings seemed to drop off quickly. Since I don't know how it works, I don't know what that means. Do cam girls set their own rates (credits per minute) or does everyone charge the same rate? I imagine that some cam girls basically live in front of their camera 24/7 and others just do it for a couple of hours per week or month. That also affects any earnings comparisons.