By looking at the median value and not the mean, you remove the impact of extreme values, for example where someone from the middle east decides they want a mint 50/1.4 lens and pays double the normal price.
One thing of note is that you tend to get clustering of prices. If someone overpays on an auction, the subsequent auctions are affected for a while. And vice versa. I was looking at buying a V500 scanner, and a chap was selling a load of them. As a result, the price dropped about 6 months ago to less than 100 including postage...its now back up again.
I must say, though, that for value for money, I think all this old kit is cheap. My brother bought a 15-85mm zoom lens from Canon...terrible distortion at the short end, which you could see through the viewfinder. It was more expensive than my entire 35mm set up. When you consider what people pay for drinks and drugs nowadays, getting out of their minds, photography is both cheaper and better for you!
I also have a Bronica ETRSi with 50/75/150 lens kit...incredible value for what it can do. However, I don't buy this stuff pretending I can make a living by selling it on! Its just valuable to me as I can take pictures with it.
One setup I'd be particularly interested in would be a Contax G1 with 28/45/90 lens kit. I think it sounds awesome. But then I'd also like a Spotmatic with 28/50/105 etc etc....