A couple of points--
1. I-Luv makes a good point, however there are basically 2 kinds of Class 3 stuff, Machine Guns and all the other stuff (Destructive Devices, Supressors, SBS, SBR, AOW's). Only the new production of MG's is banned. That is why M-16's are $6k now, as you cannot make any new ones for civilian sale since 1986. So, the .50's are likely to become Destructive Devices rather than completely banned (MG's). Once the decision is made, the .50's will jump FAST by at least 50% in price until the law/ruling takes effect (people trying to get them before they have to pay a transfer tax). Then prices will drop down a bit to slightly above current levels. If they stopped all current civilian production (like MG's), then the $2k State Arms I have will go for $4k overnight. If you look at supressors now, a $200 cheap supressor is not worth selling (as an individual) as you have to pay a $200 tax to move it off the Form 4 to a Form 3 (assuming it is an out of state sale) and pay the tax again to put it back on a Form 4 for the purchaser (easier for a purchaser to buy direct from a dealer, since only one tax has to be paid).
2. Don't go by the jump in HK's. After the '89 import ban and the '94 assault weapon ban, things changed a little bit (prices for pre-bans went up). Since then they have climbed SLOWLY. Unless we get a California style AW ban (no new registrations--so people will go crazy trying to buy one before it is too late), I don't anticipate a major price jump (and if we do, the "assault weapon" you have is worth SQUAT after the new ban--since you can never transfer it).
I would go with I_L's advice and get the .50. You can get nice guns for the $2k range (if you can come up with a little more, that gives you the choice of a State Arms from $1800 to an Armalite to an AMAC to a Barrett for $2600). Ammo and good scopes for the .50 do add a lot to the cost, however.
The other advice (aside from "collectables" that I really don't know enough about) would be to look for an FNC. You can get a nice FNC now, and a sear later to make it a full auto (FNC $2k, Sear now is $1600, 3 rd. Burst $300).
AFARR