That is very good, Nico. I work for a successful hospital, and we do good to collect 35%! Your Dad's discounts make great sense.
Thus, why on average they charge 2.5 times as much for 'uninsured' patients: 2.5*.35 = .875, or they're STILL earning less, on average, than patients with insurance. Matter of fact, they'd need to charge nearly 3 times the insurance rate to break even. (2.96)
I'm willing to bet that most of those paying the 2.5 times rate are being billed for it. Offer to pay cash, up front, and they don't have to worry about collection averages, debt collection services, etc... It's also substantially cheaper than dealing with insurance. I've heard of some dentists working after hours for cold hard cash and making some real deals, but they're probably not reporting that income.