Well, I know that in Vancouver, 30-50% is normal.
If the agency takes 30%, there might be additional advertising fees and "shift fees" etc. I even know of one agency that takes 50% and doesn't provide a driver for outcalls! (The lady has to pay)
If, as Gen mentions, the agency takes 40% and actually provides everything from photoshoots, to booking appointments, to advertising, to security and drivers, and so on, then I also agree with her that it is a reasonable deal.
Advertising, booking appointments and arranging everything takes about 2 hours out of every day for me, whether I'm making money that day or not. So I understand why some ladies would rather only work on the day they actually make money, and give an agency a cut in exchange for saving them so much time and hassle.