they've ALWAYS neen a lot of money, and in VERY short supply, even for the knockoffs-the ones made in germany by solingen( thich might be a better knife, btw) i think that 1) there were only 1million or so m14s made, so there MIGHT be 2 million bayonets in circulation
2 the m1a shares a bayonet lug( and there's a LOT)of those produced by springfield , but no blades, so that also shrinks your supply
now when you add the other manufacturers into the mix , the supply shrinks even less
that's not to mention the "shrinkage"- lost in battle, dropped, stolen, etc- it's basically a short garand bayonet with a different tail - and not everyone wants one- now the m16, on the other hand, is VERY easy to obtain, in both the m7 and m9 configs-
my own personal experience was that i bought a german knockoff for 30 bucks way back when, and then in the 90's paid 75( omg) for a battle experienced one-so it's supply and demand as well as a COLLECTOR'S ITEM, just like the line throwing kit-which we just kicked overboard when done with