Sellers:

http://www.tf2outpost.com/trade/21643631 7 at 1.5 Buds each(lmfao)

http://www.tf2outpost.com/trade/23172516 13 Keys, Young(might've kept for longer cause r+ubi relists a lot)

http://www.tf2outpost.com/trade/23139987 14 Keys, Young

http://www.tf2outpost.com/trade/22959495 15 Keys, 2 Weeks unsold

Solds:

Sold for 14 Keys

http://backpack.tf/profiles/76561198060656198?time=1406790000 -> http://backpack.tf/profiles/76561198060656198?time=1406723744

http://www.tf2outpost.com/trade/21630106 (The last comment)

Sold for 14 Keys

http://backpack.tf/profiles/76561198058636683?time=1406336655 -> http://backpack.tf/profiles/76561198058636683?time=1406358000

Sold for 14 Keys

http://backpack.tf/profiles/76561198077418580?time=1411023600 -> http://backpack.tf/profiles/76561198077418580?time=1411282800

It's clear that 15 doesn't happen anymore, though 14 Keys seems pretty consistent. 14 Flat should be fine.

Additional Proof/Counter-Proof/Constructive Criticism/Feedback is always appreciated.