I bought an HK 94 back in 85 for $450, which was the going rate in our area at the time. In 86, about a week before the ban, I shipped it off to Fleming and had it converted to an MP5 for $350.
Last I saw, HK94's were going for around $4500-5000, but thats been a couple of years ago. Not sure what they are going for now.
Of all the guns Ive bought over the years, the HK's (and their accessories) grew the most in value, and brought the most money when I went to sell them. Not too much else has come close.
About a year before I got the 94, I bought a P7M13 for $450 as well. A couple of years later, I more than doubled my money when I sold it.
I also bought a couple of HK91's a short while after the 94, both in the $600 range. Both brought over $2000 (the guns themselves, no accessories) when sold, and that was back in the 90's.
I also made out quite well on things like claw mounts, port buffers, stocks, etc.
What made the HK's worth so much, was the 89/94 ban George Bush put through. Dont think its just the democrats you need to worry about. The republicans are just as bad. HK also got a little weird about the same time and stopped dealing with the general public.