A small tip: when you need money for upgrades or anything and you're at the Agraprom Research Institute, just wait around for a while at the west entrance to the Stalker-encampment. It gets attacked about 3/4 times a day by a bunch of bandits. They're quite easy to knock down, and the weapons they carry can easily be transported to the trader. A inventory full of weapons should give you a couple of grand at a time.
As an addition to my post above, another weird thing happend: when I cleared out the little swamp area at the western-most part of the Agraprom-map, I got a 'Sunrise'-suit from one of the Stalkers I helped. It was upgraded differently than the sunrise-suit I already owned, but I decided to take it to the Stalker trader. When I put my 'original' suit on the ground to be able to make the distinction between them and later picked it up again, all the upgrades on it were gone! I spent about 15000 on that suit and they just went up in smoke. Anyone have that problem too?
So wolfman shit, eat my shorts.