US postal rates are a fairly good approximation to inflation.

first Class Stamp costs
1978....$.15
2009....$.44

Inflation factor 1978-2009 = 44/15 = 2.93

Old sleeping bag cost = $120 (assume 1978)
New 2010 sleeping bag = $120 X 2.93 = $352

Franco said that things in the old days really aren't as "cheap" as we remember.

You could look up the other inflation indexes such as the CPI (consumer price index) and give it a go yourself.

The modern bag at that price range will have lighter fabric and much loftier down so the weight per warmth will be much less or the same warmth of bag will weigh less.

Get real and buy a good bag. I managed to get a Western Mountaineering MightyLite for well under $300 by looking all winter for bargains and doing trading. I paid $250 for something new and managed to sell it for $320 and used that to buy a WM bag for less than $300. Had money left over, sorta.

Previously, I had never spent over $140 on a sleeping bag - after the usual careful shopping for long periods.

I'm very happy.