T206 Back Rarity- Cost multiplier

[Metsfan0507]

There's been a lot of interesting posts about rare back T206s, and as someone relatively new to this world I'm curious how this forum generally sees the value of a card changing based on back rarity. Let's say your standard Piedmont/Sweet Cap Back is the baseline 1x value. Is polar bear a 1.1-1.2x? People will pay a little more for it, but not much. What about the others? To get started I'm using our favorite t206resource.com back scarcity rankings and creating tiers, but feel free to disagree with the tiers I'm creating:

Tier 1: Old Mill Brown, Lenox Brown
Tier 2: Broad Leaf, Drum, Uzit, Hindu Red, Lenox Black, Carolina Brights
Tier 3: Piedmont 42, American Beauty, Hindu Brown, Sovereign, Cycle, Tolstoi, EPDG
Tier 4: Old Mill, Polar Bear
Tier 5: Sweet Cap, Piedmont other than 42

For the sake of simplicity I'm not breaking the backs out by Series, but feel free to comment if you think that certain series of a brand belong in different tiers.

Thanks!