A well made arquebus is MUCH cheaper then a well made sword. Guns are a hollow iron tube with a small hole for powder attached to a wood stock (sometimes).
Now the wheel lock mechanism is fairly hard to produce as it is essentially clockwork. But your bog standerd match fired arquebus is incredibly cheap
It is about as easy as maintaining any other metal weapon
While the near faceless mass combat of the late 17th and entire 18th and early 19th centuries is ineed damaging to the idea of the warrior ethos, the time bordering that is not. Albeit the gunslingers of the mid 19th century are of a different character then the medieval warrior. But a knight on horse back is still a, if not the, dominent force of an open battlefeild for the entire 15th century, and most of the 16th. And even into the 17th. Battles usually came down to cavalry to decide them as pike and shot formations would lock horns and whomever won the cavalry fight would be able to turn ones flank. This means you need a skilled and elite cadre of cavalrymen. The death of the warrior ethos in Europe was a centralisation of power choking out a lot of the nobility, so they were just less noticeable. And in Japan it was the fact that the nation underwent several centuries of peace. You don't need samurai when there's no one to fight.
Also, in the early days of fire arms, the high quality plate was proof from shot. In fact a pistol would usually be discharged at close range into a breast plate to prove it. As guns got better, this became less true (Which is why heavy cavalry abandonned full plate in favor of heavy breastplates), but at the presumed time era of PE, this will still be the case.
In fact, wearing super heavy full plate makes FAR ore sense in a society were early guns are common (enough), because you really don't need that much armor to stop a sword, axe, or arrow.