[lemon1324]

Those work by measuring voltage generated in a coil of wire wrapped around the wire you're trying to sense. To induce this voltage, you need an AC signal in the wire who's current is being measured.

The motor controllers output a pulsed DC signal though, so they may work (Our mentor was formerly a EE prof, and thinks pulsed DC may work through a coil of wire for sensing). We haven't tested this yet.

There are also hall effect devices that measure the magnetic field generated by the wire (and work for DC applications) and I think you can get a hall effect IC for ~$2-5