Cheap isolated mains current measurement
Depending on the cost to you of winding thin enamelled copper wire through a ferrite toroid, a practical answer may be to wind your own toroid. This is of course simply making what manufacturers sell as 'current transformers'. If you can save costs by going for only a few turns of secondary, or a small ring core, then you win.

There are two ways you can use a toroid wound like this on a current-carrying cable to detect or measure the current. The first is as a current transformer, the second is a current-driven voltage transformer. They behave quite differently.

In a current transformer, you short circuit the secondary, or at least load with a very low value resistor, the burden resistor. This keeps the voltage down, and the secondary current that flows cancels most of the primary current and so prevents the core from saturating. A 'typical' core with a decent volume secondary that's near-shorted is unlikely to saturate. The transfer gain will be consistent from part to part and with temperature, being dominated by the turns ratio. No particular permeability is required, only that it be 'high'. Cores are readily available with relative permeability from 2k to 10k.

The core flux can be calculated as follows. Calculate the secondary current from the turns ratio. Calculate the secondary voltage from this and the total secondary resistance, winding + burden resistor. Calculate the transformer volts per turn required. Now calculate the rate of change of flux (B field times core cross section) required to generate that volts per turn, and convert that to peak flux. Note this calculation does not require the core length or the permeability.

In a current-driven voltage transformer, you leave the secondary open circuit, and detect the voltage. There is no secondary current to counteract the primary current, and so a low core permeability must be used to limit the flux to below saturation. The transfer gain will depend on the core permeability, which will vary by part and with temperature. This is why measurement transformers are usually operated in current mode. This could also be regarded as a cored Rogowski Coil. 

To calculate the core flux we divide the peak primary current by the core magnetic length to give us the peak H field. This is multiplied by permeability to give us the peak flux density. The core relative permeability will typically need to be low, in the low 100s, maybe even high 10s. These cores are advertised as being suitable for inductors.

Of course with both modes, if you only need detection of the primary current, rather than linear measurement, then it doesn't matter whether the core saturates or not.

The achievable cost depends critically on your quantities. If you want 1-10, you can wind them yourself, while watching TV. If you want 100, you and your friends can make them. If you want 1,000,000, then you can commission a custom part from a toroid winding company, value engineered to your specification. If you want 1,000 to 100,000 then you're in trouble, a custom part is out of the question. The best you can do is try to get a decent quantity deal on a part that's already being made.