When with an agency, the fee is split, except for any tip.  
As an indy, the indy pockets it all.  

Let's assume 400 split 50:50. She is receiving 200 per hour but spending little or nothing on ads, little or no time on screening and booking, etc.  

If she keeps her overhead costs at or below 200 per hour (averaged out over an entire given time period), then she only needs to charge the same 400 to be where she was before.  

I'm ignoring that she might want to work fewer hours for a higher rate; that the agency room subsidy (sometimes 100%) was generous and now she is stuck with 100% of the fancy room expense; that she attracts a lot of time waster who call and email but never book. Some agencies also help with pro photo sessions but I don't know how much. (Some agency ad pics seem to get updated all at once. Even if they are not paying 100% for the girls' shoots, maybe they negotiated a nice discount to shoot several girls on one or two days.)

Some agency-to-indy girls do raise their rates but not all of them.