Why are sound level meters that measure dBZ so expensive

[FrankT]

I'm not an expert in this are so please gently correct me where I'm wrong...

I'm looking for a sound level meter that measures low frequency traffic and machinery noise.  The cheap meters <$500 seem to only measure weighted dBA and dBC which filters out low frequencies.  Meters that measure dBZ seem to be >$2k.

Is there anything special about these meters?  Is it just software or product demand, or are they made of unobtainium?

[TimFox]

Microphones with extended low-frequency response are more expensive than those that cut off at higher frequencies.

[james_s]

Low volume production too, how many people need one of those? They're not selling millions of them a year. I suspect it's more like hundreds.

[DaJMasta]

Do you actually need to be listening through the air?  You can get up to a few tens of Hz mounted to solid materials with a seismometer or similar.  They also make microphones designed for these applications (geophones), maybe something like that would suit your task but doesn't show in your searches.