MarkT:
Yes, those components look OK - the problem may be creating a gap for the hall device,
cutting ferrite is difficult (diamond wheel will do it).Or you can add the hall sensor on one side and a piece of iron on the other.
I'm not sure if I picture it right: I put the ferrite around the electric cable, on one side I put the Hall sensor, on the other a piece of iron in something wich may looks like this:
Am I right ?
You also need to calibrate it - the mechanical dimension of the gap the hall
sensor fits in is crucial as the magnetic flux depends on it and the current.
Do you have any links about the "rules" of calculations involved here ?
A CT has the advantage that the magnetic circuit is not crucial and there is
no gap to worry about.
I can't find CT under 9$ (etc: https://www.sparkfun.com/products/11005), building it myself would get me this for < 5$ 
Thanks for your time and help.