I'm curious as to why you don't find anything useful in the track record of valuations (past 90 days, for example) provided by Gunbroker. What you have there are motivated buyers who state what they are willing to pay in real terms--which is what fair market price revolves around. There is ebb and flow, of course--sometimes there are no ready buyers for MAS offered, and in that case they sell quite cheaply, other times two or more buyers get engaged in a struggle and the auction goes to 150 or 200% of normal selling prices for that one particular MAS. Again--the next one offered may be met with complete indifference and languish for months with no bids.
Valuations are sketchy. Nothing is really "worth" anything--there is only the real-world price that someone is actually willing to pay for it--i.e. actual cash on an actual barrelhead.
If you are looking to lowball (and I don't sense that you are) the local gunshop would offer him practically nothing for a MAS 49/56 which comes in off the street--maybe a couple hundred bucks with accessories, tops. If he put it on Gunbroker he may (Repeat: MAY) get $500 - $650 for a nice-looking MAS. Or it may go months with no bids.
For a happy medium--if he originally bought it when they first came appeared on the market ($189 - $229 +/-) ask him if he'd sell it for twice what he paid for it (then you should pay more for any non-standard accessories). You could probably find one cheaper if you looked hard--and he could probably find one selling for more if he was willing to work at it--but you can both shrug it away and drink a beer and call it good.
Hope this helps.