It was the only gun I could find in the dark, very, very quickly. Back then I had a bad habit of putting my carry gun wherever when I got home. That was a problem in the dark trying to find a black handgun.
Since then I'm much better prepared.
A lot of the value in a transferable MP5 is on how it was converted. Was it a registered receiver two push pin conversion, a sear that can be divorced from the host, sear pack, or? Some of the more desirable conversions are going close to $50k or more.
I got mine some years ago. I got some good advice from a friend long ago. If you find a NFA deal buy it and resell it. Study values and know what you are looking at. I had some friends to ask questions on stuff I did not know. I got a 401k loan and turned my profits into what I really wanted...