Ouch!
Well, burning the power up with resistors would mean at least 3 watts for a 15mW output. You won't find a regulator rated for anything like 600V on the input. In any case, a simple 3V regulator may need 3 mA just to idle, so efficiency gets worse. Maybe a 3V zener and a 120k resistor would work ok.
There are ways to do a far more efficient dc/dc converter, but you won't be able to make it work without a complicated bootstrapping system (there's no voltage to run the converter's logic until the converter's running).
Maybe a resistor is the best solution, but what are you trying to do? Is extremely poor regulation, no surge capability, and poor efficiency ok?