Figuring T206 pricing upcharge based on backs?

[chalupacollects]

Here is a question I haven't seen asked...

Ran across a gentleman today that had a small stack of T206's for sale... None graded... Claimed to be a "big collector" with 3 storage units of cards...

Anyway the ten or dozen cards he had with him were all Polar Bear backs with a couple of Sweet Caps and Piedmonts mixed..

The Polar Bears included included a Cobb on shoulder, Christy Mathewson, Chick Gandil, Clark Griffith and a few other stars.

I didn't examine out of the top loaders but they looked ok but being some big names and not graded, I was very leery for someone claiming "big collector" status..

So obviously he had some big numbers on some of them, $4500 for Cobb etc.

The question is if say using a Beckett Vintage guide as a starting point - I know not the greatest guide but it is at least a starting point for a price...

What percentage would you add to it or rule of thumb to use for pricing the rarer backs such as Polar Bear, Drum, Uzit etc.

So if a Gandil lists at $175/300 for a common back, where would you be on a Polar Bear?

Thanks!