I don't have anything for you.
I checked out isthereanydeal dot com, and the lowest it's ever been on sale is $15.
By offering $15 you are not providing a very strong incentive to purchasers of the game to give up their extra copies.
Someone would have had to have bought the game on sale at its lowest price and then decided some day (?) that they just didn't want it anymore.
The website also pointed out that it's available for $15 on some website I've never heard of, so maybe that's up your alley.