Sale 1:

Sold for $36.48 on Marketplace.tf

https://marketplace.tf/items/30018;5;u35

https://backpack.tf/item/4625856522

36.48/2.07 = ~17.6

X = 18

Sale 2:

Sold for Miami Nights Elf Esteem (13.5) + 2 keys

No outpost

Seller: https://backpack.tf/profiles/76561198179947127#!/compare/1499558400/1499731200

Buyer: https://backpack.tf/profiles/76561198198332539#!/compare/1499644800/1499731200

Rounding down, due to the 16 day old seller at 12 on the Elf.

X = 15

Sale 3:

Sold for 13 keys + 15 ref (<0.5) in ~2 days

Seller: https://backpack.tf/profiles/76561198198332539#!/compare/1499731200/1499904000

Buyer: https://backpack.tf/profiles/76561198312007577#!/compare/1499817600/1499904000

X = 13

Sale 4:

Sold with Strange Merc’s (~2) for 12 keys + cheap cosmetics?

Seller: https://backpack.tf/profiles/76561198175216999#!/compare/1499644800/1499817600

Buyer: https://backpack.tf/profiles/76561198091685465#!/compare/1499731200/1499817600

This sale will be way too low. Excluding it.

Sellers:

18 keys:

http://i.imgur.com/f2c5EQK.png (2x)

20 keys:

http://www.tf2outpost.com/trade/28989335 - 2 months

Sales at:

13

15

18

13 sold quickly. Excluding it.

Going with 15-18.