Yes, that is certainly a lot of money considering the gun - as shown - is certainly not in good condition and there are many parts missing.
Also, given that in "Kill Straker!" there are - at least - two other 'identical' guns shown in the 'armoury' visible starting at 40mins 31secs there is surely no guarantee that the gun sold is the one that Paul Foster (Michael Billington) was actually using in the scene.
At 44mins and 44secs Foster grabs one of the two guns from the gun rack I mentioned earlier.
One other thing I noticed: as Commander Straker walks into the shooting gallery at 40mins 28secs he walks past a gun rack - to his right - and the two longer guns at the centre 'look' on closer blowup examination could be the same type if rifles as well.
I have never noticed before, but there are two entrances and exits to the gallery:
#1 at 44:23 etc. and the other to the right of the 2nd gun rack...
#2 at 44:13 and onwards
And, of course, a very similar if not identical rifle is used by the SHADO guards who pursue Foster in 'Court Martial'. So, yes, even if it is a genuine prop, it may not necessarily be the one used by Mike Billington. I can't see how this could be 100% proven. But there must be a 50% chance at least. I have to wonder where this gun has been for the last 49 years...?