Those prices ($15,000-18,000) were for fixed wing. It's not cheaper than helicopters because of the longer distances (read as: greater fuel burn) unless you're looking at a time based or distance based (per mile) rate. Even with a turboprop, going from Indy to Florida is going to set the company back about $6K-7K for fuel and other expenses.
I've done many from near Buffalo to just north of NYC. The company had the interfacility transport contract for NYS Dept of Corrections. Those were some interesting transports, as they were all female, maximum security inmate. Approx 390 miles one way.
The current employer routinely does Rochester NY to Cleveland OH (270 miles), Pittsburgh PA (290 miles), and NYC (375 miles).
My company just did a trip from Central New Jersey to South Carolina. They at least paid for the hotel after we dropped the patient off. That was the longest trip I have ever done and I ll never do it again.