I don't really see how there can be a multiplier that can be applied across all sets. Even in postwar, there's a lot of variation. Callahans were sold as sets and the most common grade is an 8, so a seven should be a bit cheaper than normal. 1950 Drakes are pretty tough in anything over a 6.
Individual cards within a set can be harder to find in nice condition too, so I might pay more for those in a lower grade.
In prewar, the same stuff happens. There's a few T206 commons I've bought for more than the grade would seem to deserve because the front/series combination isn't common.
Some sets I'd be very happy to find any card at a price I could afford.
What I do is make a rough comparison to recent sales on Ebay. That sort of gives me the going rate for a particular card or typical common.
Steve B
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