Paid 400 for just a rusted M11 lower receiver a few years ago.
Owner had a basement flood while on vacation for a couple months and the house sitter never went downstairs!
Mold everywhere, almost everything destroyed, while helping move his stuff upstairs to the garage(he was throwing everything out),
I saw the gun and offered what I had in my pocket at the time 400 bucks cash(plus 200 tax, was a steal I thought)!
Built new receiver, and cut old receiver serial number/proof marks off and welded to new one.
It was a bitc but worth it. It would have been easier to just engrave a new serial number but I was told that was illegal.
Anyways, the rust was so bad, there was just no way to refinish the old frame.
All in all with accessories/conversions and a collection of 50rd stick/ 200rd drum mags, you can say Ive spent at least 5k on it anyway.
The point is that, yes 5k seems like a fair price, as I may have just spent that on the frame myself instead of doing all the work lol.