I like shooting them more than I know about them and will do my best to try explaining this. Seen some other guys on this forum that have an excellent grasp on the subject, but they can correct me where I'm wrong and fill in the blanks.
Looks like you are confusing the assault weapons ban of 94' with the machine-gun ban of 86'. Machine-guns are exempted from the 94 ban. Ninety four ban was for title one weapons and machine-guns are title three. In fact, if you happen to come up with the money for a registered auto sear (or DIAS, drop in auto sear. Same beast!), it can be installed in a post ban (94') title one gun and every dark feature that crosses your mind can be done to the weapon, legally. SBR (short barreled rifle), flash hider, bayonet lug, folding stock. Virtually no limits. Assuming we're talking AR's still.
Now, the 1986 ban is a machine-gun ban. Among other things, the law placed a ban on the future production of machine-guns for civilian use. So in other words, there are a finite number of machine-guns across the country. Unless the ban is lifted or another amnesty period is granted and there will be no more than those X number of guns. That is why they are so expensive and desirable.