I don't know if this answers your question or not but here goes...I'll use both Lou Gehrig and Babe Ruth as examples here, as they have a bit in common from an autograph standpoint. For a long time the prices for an autograph of either inched skyward and (I think) peaked about 7 years ago. There was a time that it was nearly impossible for me to have either a Ruth or Gehrig autograph in inventory for more than 24 hours.
Since then I've noticed a steady slow decline in pricing on both. In general either one of them sells for about 20%-30% less than they did back then. Its not the economy or the hobby (I don't think) as while they are selling for less some other players autographs are selling for 20-30% more than they did 7 years ago.
The exception being the really choice and unique Ruth/Gehrig items are STILL going to fall into the "skys the limit" range but as an overall rule of thumb the prices have come done.
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Buying and selling Vintage Autographs for over 3 decades
jim@stinsonsports.com