Last suggestion was based on an unsold. Updating with sales. This is also a change bigger than 15%, so this should be fine.

Sale 1:

Sold for 14 keys

No outpost

Seller: https://backpack.tf/profiles/76561198048851745#!/compare/1501372800/1501545600

Buyer: https://backpack.tf/profiles/76561198108318537#!/compare/1501286400/1501545600

X = 14

Sale 2:

Sold for 13 keys in ~5 days

http://www.tf2outpost.com/trade/29305963 - B/O of 14.5

Seller: https://backpack.tf/profiles/76561198108318537#!/compare/1501977600/1502323200

Buyer: https://backpack.tf/profiles/76561198074283200#!/compare/1501891200/1501977600

X = 13

Sellers:

13 keys:

http://i.imgur.com/vhdouXs.png

http://www.tf2outpost.com/trade/29313914 - 1 week, was probably lowered recently as the classified listing was re-listed recently.

Sales at:

13

14

Going with 13-14.