Most here have probably already done this, but for those who haven't, a useful link is:
http://www.westegg.com/
A bike that cost 500 bucks in 1972, would sell for $2280 today, adjusted for inflation
A bike that cost 800 bucks in 1975, would sell for $2996 today.
So, inflation alone accounts for the value of *most* vintage lightweights. Those who have been shocked by the rise in price of many Masis from the 1970s might be interested to know that the bikes have been undervalued, adjusted for inflation. It's only in the last couple of years that the bikes have started to catch up to inflation...partly as a result of interest generated by this list.
As for that De Rosa on ebay... inflation clearly accounts for some of the value, and if you take away the top two bidders, the price almost looks rational. But who knows how many others bid on that bike at the end, whose bids didn't show because the ebay server rejected them, because the top-bids were already in place? There's a huge premium for desireability and rarity in the case of that bike.
Most vintage lightweights in clean condition are undervalued even now, adjusted for inflation alone, and not counting any collectibility premium.
Charles Andrews SoCal
"What concerns me is not the way things are, but rather the way people think things are."
- Epictitus