Sale 1:

Sold for 12 keys

http://www.tf2outpost.com/trade/27739832 - B/O of 12

Seller: http://backpack.tf/profiles/76561198027125031/#!/compare/1453363200/1453449600

Buyer: http://backpack.tf/profiles/76561198104058879/#!/compare/1453449600/1453622400

X = 12

Sale 2:

Sold for 10 keys

No outpost trade available

Seller: http://backpack.tf/profiles/76561198172446667/#!/compare/1452844800/1453190400

Buyer: http://backpack.tf/profiles/76561198025618418/#!/compare/1453104000/1453190400

Compare links weren't really effective this sale, so I've provided a link to each key on outpost.

Keys were traded from Mudrik to Australium Man

http://www.tf2outpost.com/item/440,4514360532

http://www.tf2outpost.com/item/440,4514360535

http://www.tf2outpost.com/item/440,4514360539

http://www.tf2outpost.com/item/440,4514360543

http://www.tf2outpost.com/item/440,4514360545

http://www.tf2outpost.com/item/440,4514360550

http://www.tf2outpost.com/item/440,4514360553

http://www.tf2outpost.com/item/440,4514360559

http://www.tf2outpost.com/item/440,4514360565

http://www.tf2outpost.com/item/440,4514360572

X = 10

Sale :

Sold with Nuts Juju for 17 keys

Seller: http://backpack.tf/profiles/76561198080410236/#!/compare/1453708800/1453968000

Buyer: http://backpack.tf/profiles/76561198073682619/#!/compare/1453622400/1453795200

This looks like a bit of an outlier

X + 13.5 = 17

X = 3.5

Yeah, it's an outlier.

Sellers:

12 keys:

http://puu.sh/nS0xG/01af288a71.png - 1 month

Pure sales at:

3.5

10

12

Going with 10-12 (current price is 11 flat)