I believe one consideration often neglected in comparing the operating cost of R22s vs. 300s is the comparative ease of rotor overspeeds in R22s in autorotation training.
This cost mostly applies in a busy training environment, with new instructors teaching new students, and can be minimized with good CFI training on how to recognize student mistake processes and recover without letting the rpm wind up over the red.
I've never had the pleasure of giving instruction in the 300 series, but, having recieved my initial training in one, I don't remember having nearly the trouble controlling the Nr as most students I've trained in R22s. I have seen several R22s go out of service due to overspeeds at much cost in mx. and lost revenue time.
If anyone out there has time teaching primary students in both types, perhaps they could comment on this.