After hearing about the time it takes to set up a machine for a certain task, perhaps the only way to make any part cheaper than others is to dedicate your machinery to that 1 part.
Keeping that in mind, I wonder if demand for 98 Mauser bottom metal assemblies is substantial enough that custom gun part makers would buy straddle floor plates for them (maybe Springfields?) if they could buy them already made up at a cost cheaper than they can produce them? Looks like tons of machine work there.

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I have a little, piss ant, 9X32 inch column mill that does nothing but drill port holes in muzzle brakes. It has been set up with a small dividing head and tail stock for 6 years and the setup has never been torn down. I actually went to tear it down for something else once and as I moved the wrench close I swear it made a funny squeaking noise, started moving toward the door and leaked coolant on the floor!

Not a word of bullshit and it wasn't even plugged in.