There are actually more variables with respect to revenues on documents than postage stamps on cover. While both have variability with respect to rates paid, solo vs. combo usages, cancels, places of usage, etc., you now add the transaction/document type and the companies in question to the mix.
I don't see it ever happening.
Your best bet is to look at sold listings on eBay
, Eric Jackson's website, and my website to get a feel for what constitutes premiums, but even then, minor differences can result in large shifts in value.
As with cancels, the right document can turn a $1.00 SCV item into something worth hundreds of dollars, and Scott will never reflect that, nor does it for front-of-book covers.
It takes time to build up that knowledge/experience; there are no easy shortcuts, unfortunately.