1909S-1MM-002 : also known as the S/horiz. S. These are actually pretty common for 1909S cents, and are only worth a small premium over the 'normal' coins. Most price guides list them on a separate line, and not for a whole lot more money. For instance,
Coins Magazine (Oct 2008 issue) lists the 1909S in VF for $170 and the S/S horiz. for $190.
The one to find is RPM#1 where the S/S is a northeast spread, quite strong, and scarce. That coin (according to the price guide above) would be over $200...which is good for a key date coin.
Biggest problem with die varieties on key date coins is that the premiums are pretty low for the die variety because the value of the normal coin is so high to start with. Generally premiums for the die varieties in the keys run in the 10%-15% range, while premiums for die varieties on common date coins can run into astronomical figures...10,000% premiums are not unheard of. Look at the 1917
DDO cent - a VF normal coin is $2, while a VF
DDO is $325 (16,250% markup).