It seems there are other interpreters who will work for a lot less than what you charge, how do you explain the difference?

In the field of interpreting, you truly get what you pay for. Interpreters who compete on price usually tend to be relatively new to the field. They are trying to find a market for themselves. No doubt, some are well qualified. However, when it comes to interpreting, experience truly rules. There is often a direct correlation between an interpreter's experience and the fees charged. Heck, I used to charge $12 per hour when I started out (my hourly rate today is $150.)