Okay, it's /k/ time!
Even though it's stamped steel, there's still milled/machined parts involved, and since the Self-Defense Forces was the only legal customer, Minebea had to do one of two things:
1. Set up an expensive factory/production line specifically for the few thousand PM-9s being built for the JSDF; either they or the government eats the cost of tooling and machinery for what is essentially a very limited-production weapon.
2. Build each weapon by hand and forego having a factory line at all.
Since Japan makes it illegal to export "defense articles" like military weaponry (and even the JGSDF's particular camo pattern), they had no other market except the limited Self-Defense Force contract, which only allowed for a few thousand PM-9s. Whenever you have such a limited production run, prices jump.
tl;dr: No mass production due to limited market, cost goes WAY up.