See
http://goscf.com/t/86916 "Sometimes Treasure Can Be Found".
A tattered and ordinary looking cover from 1911 with a starting bid of $1.95 on eBay sells for $5500.
Less than 20 of this "Orangeburg Coil" are known to exist on cover, and it has a catalog value of $25,000. Not a bad day for the seller who thought it was just a dollar-box cover.
My best kiloware find was a used US 22 cent "Stamp Collecting" booklet single 2198b with black color omitted. The unused missing color booklet pane is $50, I don't know what a used single is worth, but it's gotta be more than the 2 or 3 cents that I paid for the stamp in the kiloware.
(p.s. 3193zd, I've always wondered what your avatar was about, and now I understand. Thanks for the explanation).