Pre Auth for DVD Rental machines

Yet another topic that has come up that I figured we might as well chime in on. Pre Authorizations for our Kiosk machines. The question is and still is, "How Much"?
I have mine set at $10.01. I like the 01 because it sometimes gets the folks to call me. I use the phone call as an opportunity to explain the machine and the way it works. Most of the folks I talk to turn out to be really great renters. Some do a full pre-auth and others do a partial. It's a tough deal because some folks don't have much in their accounts... What do you guys do?

Sincerely,
Mike Weiland
http://readydvd.com