Short term (say 10years or less) they are like anything else you buy at retail. Over the long term they seem to have kept up pretty well with inflation. 1973 Gun Digest prices:
S&W Model 29---$194
Colt Woodsman---$120
Ruger Mark 1---$69
Colt Diamondback---$140
Colt Mk IV, Series 70 45ACP---$135
Dan Wesson Model 15---$105
Colt SAA---$195
Ruger Blackhawk---$99.50

This was MSP-you could probably actually buy them for less.

I believe any of them would now be worth many times that cost, as used guns