For this reason:
They are so cheap because the number of potential legal purchasers is so small.
To my knowledge, there aren't any pre-1986 Glock 18s out there, which means none are on the NFA full auto registry, and that in turn means regular folks can't own them.
With such a small group of potential buyers out there, the 18 ends up sounding insanely cheap. However, you see much the same thing with post-86 dealer sample full auto guns; they also aren't trading at anywhere near the prices of the pre-86 transferable guns.
Get rid of the Hughes Amendment that closed the full auto registry, and you'll see M16s going for not much more than an otherwise identical AR-15, Glock 18s going for $600 NIB, and all kinds of other surprisingly cheap full auto guns. I'd expect the ATF would need to expand the NFA branch considerably to handle the rush of applications of people falling all over themselves to pay a $200 tax.