How 'bout a non emotive benchmark to start with? Forget for a minute perceived value, recent eBay prices, rarity or it's what turned you on at age 16 and you couldn't afford it. Start with what it cost in say 1969 dollars and what it would cost in 2007 ones.
So let's just assume a PX-10 cost $210 in 1969 (I think that's pretty close actually). In today's (2006 is the latest year avail of course) dollars that's $1176. And a Cinelli SC in 1973 was what, oh let's say $550 (again a pretty close estimate) or $2560.
http://www.westegg.com/
Hmm. I bought my PX-10 on eBay back in 2002 for $450 I think. And I'd suggest the average price an ordinary white PX c. 1969-74 fetches now is still about the same in good, original condition. I bought my Cinelli SC, completely restored in 2003 for $1600. I think that was a tad on the low side. I'm sure not sharing with you guys stuff I paid way too much for!
So do the math with what you've got, of course you need to ascertain the retail price when new but that's not too difficult. Compare to what you paid now. And I'll bet you dollars to donuts that 90 per cent of this stuff isn't even beating the inflation index. "Honey, it's an investment, remember". Hmmm....
Peter Kohler
Washington DC USA