Sale 1:

Sold for Spec. KS Australium Grenade Launcher (~15 at the time)

Seller: https://backpack.tf/profiles/76561197964171516#!/compare/1493337600/1493424000

Buyer: https://backpack.tf/profiles/76561198172888683#!/compare/1493424000/1497571200

X = 15

Sale 2:

Sold for 12 keys + 22 ref (>0.5) in ~4 days

No outpost

Seller: https://backpack.tf/profiles/76561198304138933#!/compare/1492473600/1492560000

Buyer: https://backpack.tf/profiles/76561197964171516#!/compare/1492473600/1492560000

X = 13

I just so happen to have sold the exact same grenade launcher for 15 keys, to prove its value:

Seller: https://backpack.tf/profiles/76561198021055079#!/compare/1497312000/1497398400

Buyer: https://backpack.tf/profiles/76561198238626850#!/compare/1497312000/1497398400

Sellers:

17 keys:

http://i.imgur.com/3Dk5qLt.png - 3 days

https://backpack.tf/u/76561198131555957

http://www.tf2outpost.com/trade/29242236

Sales at:

13

15

Sale at 13 should be long enough to be included in the range.

Going with 13-15.