EXHIBITS help please

[shakes]

In 2007, I inheried 128 1928 (and I assume 1929) EXHIBITS boxing cards from my grandfather. I have looked at them a few times over the years but have always come to the conclusion that they aren't worth much...maybe $10-$30, depending on the card. There are Dempseys, Schmellings...the regular cast of characters.

But recently, I noticed a listing on Ebay for a "doubleheader" card for 1k! And another for $500. I get these are listings and not sales. That said, upon further digging, the mutli-colored "doubleheaders" do seem to be on the rarer side.

So my question is this....how rare? Is there a database for identifying the # of known cards for each permeantation?

I've read that "lavender" is the rarest color on a doubleheader but have no way of verifying that claim. I have 12 of them, so it'd be cool to know. This way I can treat them accordingly. Order proper over-sized sleeves, perhaps grade them,......I'm just grasping for straws at this point, as there simply seems to be a dirth of information on these particular cards.

Thanks!