How to build very low cost current transformer?
I think you'll have a great deal of trouble getting good enough performance at 50Hz from a powdered iron core toroid- the inductance won't be high enough. That's why tape-wound permalloy cores are typically used.  

If you use a large powdered iron core (to give more room for winding) and put several turns through the primary rather than just one, you might be able to get acceptable performance (5 turns would give you 25x the inductance). Start with your requirements (minimum frequency, minimum current resolution) and do some transformer design calculations. Saturation needs to be considered too. 

If you want to use air core coils, there is always the Rogowski coil, which provides an output you can integrate to give current.