I don't provide half hours as I don't really find that it's worth messing with my school schedule for a 30 minute engagement. I have 5 classes and labs to attend to, as well as lots of readings.
I only see one (maybe two) clients a day, so I structure my rates to fit my financial budget (savings, fun money, etc). I also find that when I was charging $200 an hour I was attracting more gentlemen who didn't really fit my ideal client profile (less than mannered, a little too rough around the edges). I also accept that with my higher rates I am also expected to provide a more elite level of service. I don't believe that I am 'allowed' to have a poor or moody day with a gentleman, I would prefer to reschedule so as to be at my best.
In the end I don't think that any rate is unjustified, whether too high or too low as per market value. We sell a unique service which includes a more intimate and personally invasive experience. I wouldn't argue that a lady charging $500 an hour in Victoria/Vancouver is selfish or overvaluing, she is completely allowed to put whatever price on her body that she chooses. It also is the same for providers who do charge $150 an hour. Capitalism is a fun thing and as a 'consumer' the wealth of different 'services' that span from $80 to $500 is plentiful.