What is the voltage drop for 500 meters 1.0mm copper cable drawing 230ma with 24volt supply?

The resistance of 1 mm (diameter) solid copper is about 21.8 ohms per kilometer.

That's 10.6 ohms in 500 meters.

When 230 mA flows through 10.6 ohms, the voltage drop is

I · R = (.230)x(10.6) = 2.438 volts.

The voltage of the supply is irrelevant.