Sale 1:

Sold for 15 keys + >0.5 keys in metal

http://www.tf2outpost.com/trade/29206495 - Confirmed here

Seller: https://backpack.tf/profiles/76561198094162508#!/compare/1496275200/1496361600

Buyer: https://backpack.tf/profiles/76561198107026626#!/compare/1496275200/1496361600

X = 16

Sale 2:

Sold for Disco Beat Down Strontium Stove Pipe + 4 keys

http://www.tf2outpost.com/trade/29317968 - B/O of 15

Seller: https://backpack.tf/profiles/76561198253093948#!/compare/1503014400/1503100800

Buyer: https://backpack.tf/profiles/76561198122002056#!/compare/1502668800/1503100800

Buyer at 10 on the Stove Pipe, so it cannot be less than 11. Therefore this sale cannot be less than 15.

X = 15 (Capped by B/O)

Sale 3:

Sold for 15 keys

No outpost

Seller: https://backpack.tf/profiles/76561198002516835#!/compare/1498003200/1498089600

Buyer: https://backpack.tf/profiles/76561198153402861#!/compare/1498003200/1498089600

X = 15

Sale 4:

bulk for that thing

Seller: https://backpack.tf/profiles/76561198098218334#!/compare/1501977600/1502064000

Buyer: https://backpack.tf/profiles/76561198198392589#!/compare/1501545600/1502064000

Sale 5:

bulk for another thing

Seller: https://backpack.tf/profiles/76561198198392589#!/compare/1502409600/1502928000

Buyer: https://backpack.tf/profiles/76561198044203779#!/compare/1502841600/1502928000

Sellers:

14 keys:

http://i.imgur.com/4O9bNJK.png - 1 day

http://i.imgur.com/y96XNIt.jpg - 8 days

14.5 keys:

http://i.imgur.com/0LUDz5E.png - 5 days

15 keys:

http://i.imgur.com/G3TmHCu.png - 6 days

15.5 keys:

http://i.imgur.com/hRvtUU1.png - 1 month

15.7 keys:

http://i.imgur.com/JyD8M2K.jpg - You have to be a premium TF2Outpost user to see this B/O!

17 keys:

http://i.imgur.com/OGwWiHQ.jpg - 14 days

http://i.imgur.com/xSbAfiD.png - 1 day

18 keys:

http://i.imgur.com/hgT68Nx.png - 5 days

Sales at:

15 (2x)

16

Sellers at 14.

Going with 14-16.