Current sense amplifier for high voltage (100-150V) low current (20 mA) application
This could easily and safely be done with a small Hall effect current transducer and there are many manufacturers.



Figure 1. A selection of current sensors.

They come in a variety of shapes and sizes and, while they resemble AC current transformers their principle of operation is quite different.


They require DC power.
They give out a low-voltage signal (rather than a current) proportional to the 
primary current.
Like a CT you can wind multiple turns through the device to increase the sensitivity. For example, if you find a device with 200 mA full-scale sensitivity then wind 10 turns through it for full-scale at 20 mA.


Note that the devices with three or four wires emerging from the centre of them can have the wires connected all in parallel for a high-current, single-turn arrangement or in series for low-current, 3 (or 4) turn, or any combination you can come up with.