I don't think there's any individual collectible items that I've bought for ridiculously cheap money, but the best deal I've had was buying a big box of noise (25-30 items) for 50 euros that contained, among a few other things, Merzbow - Hannover Interruption, Merzbow/Emil Beaulieau live LP, CCCC - Amplified Crystal LP, Sutcliffe Jugend - We Spit on Their Graves CD + shitload of (common) stuff I didn't have yet then by Killer Big, LHD, Controlled Bleeding, Outermost, Cracksteel, Knurl, Solmania, etc. I guess nothing in there was truly 'collector's item' in the sense of fetching 100+ individually, but the value of these things together was much more than 50 euros. Also bought As Loud as Possible from the same guy for 10 bucks which I thought was a pretty sweet deal.