Non-Invasive High Power DC current measurement
If you have access to both ends of the cable, then the voltage drop across the cable is easy to measure. 

I have been known to use a the screen of an RF co-axial cable as a heavy current conductor (some cables are built with a lot of copper in the screen), and use the inner to sense the voltage at the far end. This gives a nicely screened differential voltage at this end to measure. An op-amp used as a comparator to sense the mV can be very inexpensive.

An improved Hall sensor method would be to use a pair of Hall sensors, one attached to each side of the cable, facing in opposite directions. This way, the magnetic field due to the current would add, and any external fields would cancel, yeilding a much better signal to interference ratio.