I've been buying vintage A's for over 20 years. All that time its been rare to see a 60's A go for more than $125-$140. And the ones selling for $140 were few and far between and generally not worth it.
Some actual recorded data to ponder when thinking about how prices may have changed, how variable they actually are, and to what extent certain price changes are recent:
18" A Zildjian 70s $139.50 12 bids eBay 3-Jul-05
A Zildjian 20" ride mid 60s poor listing $90.00 1 eBay 21-Jan-06
A Zildjian 19" med thin ride 1969 kit to date it $143.00 17 eBay 24-Jan-06
18" 40s A Zildjian (pre trans) 1458g $335.00 BIN eBay 10-Jan-07 sound file
A Zildjian 60s light sizzle ride 2035g (hazelshould) $330.00 23 eBay 14-Jan-07
A Zildjian 60s 21" sizzle ride $261.00 16 eBay 15-Jan-07
A Zildjian late 50s 20" thin ride 1985g $317.00 14 eBay 10-Feb-07
Just a quick dump. I've got hundreds more recorded from 2007 to 2014.
This is why I say the variation is great (and has been). This is also why you need reasonable sample sizes to really get reasonable estimates for expected price range.
Part of the issue we are having here is an old one in marketing. There is no "underpriced" or "under valued" class of cymbals out there waiting for you to make a profit by buying low and selling high. If there were somebody would have spotted it by now and already have made the profit. There are individual cymbals which may be underpriced and you can snap them up and try and sell them at a profit. But there won't be a general category of cymbals like that.
Good point Mark that buying in some (off eBay) places may also get you 50s cymbals for lower prices because you are not competing with collectors. That's another way the market is micro differentiated: by selling channel.