I was out at one of my local gun stores and was looking at a used Ruger MK II pistol on consignment when the salesman brought over a box of goodies that came with the pistol. The one thing that caught my interest was this wire shoulder stock that you could hold when you held together with the grip of the pistol. It never permanetly attached to the pistol so it didnt make it a SBR but it was something I have never seen before. What is something like that worth?

Don't know what its worth, but stay away from it.   The ATF can still get you on 'constructive intent' or whatever they call it.   Bottom line is if you want to stay in the clear, don't even own the wire stock while owning the pistol.

EDIT... its called 'constructive intent'

There were a coupleof things like this in the '60s.
One had a bracelet with a socket to receive the front of the butt, the other was sort of like old Colt cap & ball buttstocks but were held by grip pressure of the shootong hand rather than a clamp.
ATF was not amused and bolh products were withdrawn. Sort of like the Accelerator 10/22 stock that enabled bump firing, except the 10/22 deal was initially approved, units were sold before it was declared a machinegun.
Do NOT accept that as part of the deal and don't take it home, or even out of the store. In fact, if you handled it , wipe it down for prints.

IE: it's worth 10 years and/or 250K or whatever the penalty is.

They were sold in the '80's––and were considered legal because the only thing holding it all together was your hand.

Don't know if they stopped selling them due to legal concerns or that they just weren't selling––-you really should look into that before messing with it.