Sale 1:

Sold for ~10.5 keys in taunts + Bill's (~1.5)

No outpost

Seller: https://backpack.tf/profiles/76561198163938639#!/compare/1494720000/1494806400

Buyer: https://backpack.tf/profiles/76561198199185042#!/compare/1494806400/1494892800

X = 12

Sale 2:

Sold for Terror-Watt Madame Dixie (Suggestion at 11)

No outpost

Seller: https://backpack.tf/profiles/76561198346113055#!/compare/1501200000/1501286400

Buyer: https://backpack.tf/profiles/76561198332704108#!/compare/1501113600/1501200000

X = 11

Sale 3:

Quicksold for 10 keys + 6 ref

http://www.tf2outpost.com/trade/29233368 - Seller confirms sale (Legend)

Seller: https://backpack.tf/profiles/76561198365923475#!/compare/1497657600/1497744000

Buyer: https://backpack.tf/profiles/76561198086733796#!/compare/1497744000/1498089600

X = 10

Sale 4:

Quicksold for 9 keys

http://www.tf2outpost.com/trade/29218534 - B/O of 9

Seller: https://backpack.tf/profiles/76561198147097380#!/compare/1496880000/1496966400

Buyer: https://backpack.tf/profiles/76561198191878065#!/compare/1496793600/1496880000

X = 9

Sale 5:

Sold for Violet Footprints Genuine Horace (Mini at 5 below) + cheap cosmetics

No outpost

Seller: https://backpack.tf/profiles/76561198332704108#!/compare/1501200000/1501286400

Buyer: https://backpack.tf/profiles/76561198154941386#!/compare/1501200000/1501286400

X = 5

Terror-Watt Madame Dixie Suggestion:

https://backpack.tf/suggestion/59802400cf6c7519ad726b32

Violet Footprints Genuine Horace Mini:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198130503112#!/compare/1497312000/1497398400

Buyer: https://backpack.tf/profiles/76561198161338106#!/compare/1497312000/1497657600

X = 5

Sellers:

11 keys:

http://i.imgur.com/Y3hu0DI.png - 3 days

12 keys:

http://www.tf2outpost.com/trade/29300603 - 2 days

Sales at:

5

9

10

11

12

5 is probably an outlier. Excluding 5.

9 and 10 quicksold. Excluding 9 and 10.

Going with 11-12.