Maybe I am bad at searching but it seems that even high-voltage, isolated current/power sensing click boards are limited to below 100V - for example, even something like this:
described as "PWR Meter click is a power measurement Click board, capable of measuring voltage and current through the load, connected to either AC or DC power source" with MCP39F511A datasheet suggesting 1000:1 input voltage divider has only a 100:1 divider installed and "The power supply should not exceed 50V" comment in the product's page...
I understand - safety, prototyping - but measuring (for example) power factor at secondary winding of mains transformer seems atypical (especially in the context of recycling some old hardware)...