My Vector was 1500. It jammed after the first mag, so I swapped out the extractor, ejector, and the two assoicated springs.
I noticed the locking piece was a Series 80, from the 9mm PDW, which Vector said they ground down to work. Not too reassuring, so I got a 10 mm high.
So far, it's about 300 bucks in parts, and it runs fine now. But the accuracy sucks (foot groups at 50 yards) so it's off to get worked on by Turner Fabrications. Another 250.
I'll have a little over 2 grand in it once done. If it shoot accuratly, then I'm 1500 lower than the amount you listed. If not, then it's off to the chopping block.
As for if 3500 is a lot/too much for a running, effective MP5/40 or 10, only the owner can say. Does it do anything a $1000 AR does? Not really. But it's neat to have and play with. Plus a 10mm round would really ruin someone's day. I wanted one from back in the 80's when they came out for the FBI just before the gun ban put the breaks on HK selling any to the civy market. A SBR model would be a great house cleaner, and not have either the flash and blast of the AR's, or need a suppressor taking the lenght back to rifle size.