If a WG has a pricing scheme like that I won't even consider her. I see it that she either doesn't like 30 mins bookings or wants to do less work in an hour booking, e.g. talking, making cups of tea, eating cake etc.
The way a lot of them see it is that either way they are going to be shagged, better to get a one hour booking and spend 30 minutes talking and wasting time.
I find that 45 minutes is ideal, no rush and no wasted time.