How much do those things cost?

I don't know, but Wikipedia has a good example of a NLJD countermeasure introduced in the 60s:

https://en.wikipedia.org/wiki/Nonlinear_junction_detector#Co...

(specifically the CIA's SRT-107: http://www.cryptomuseum.com/covert/bugs/ec/srt107/index.htm)

As an aside, the entire website for that second link is pretty interesting.

1688.com is your friend