Using last month.

Sale 1:

Sold for $34 on Marketplace.tf

https://marketplace.tf/items/30920;5;u3006

34/1.97 = ~17.3

X = 17

Sale 2:

Sold for 16 keys

Seller: https://backpack.tf/profiles/76561198196573375#!/compare/1523491200/1523577600

Buyer: https://backpack.tf/profiles/76561198188351270#!/compare/1522281600/1523577600

X = 16

Sale 3:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198155156950#!/compare/1523404800/1523491200

Buyer: https://backpack.tf/profiles/76561198385366296#!/compare/1523404800/1523491200

X = 11

Sale 4:

Sold with 3 keys for Mega Strike Rancho Relaxo (Suggestion at 14)

Seller: https://backpack.tf/profiles/76561198188351270#!/compare/1523664000/1524182400

Buyer: https://backpack.tf/profiles/76561198063640894#!/compare/1524096000/1524182400

X = 11

Sale 5:

Sold for Fountain of Delight Disco Fever + ~10 keys in items

No outpost

Seller: https://backpack.tf/profiles/76561198063640894#!/compare/1524182400/1524268800

Buyer: https://backpack.tf/profiles/76561198013292193#!/compare/1524182400/1524268800

Gonna be too high here, given the current sellers.

Mega Strike Rancho:

https://backpack.tf/suggestion/5aef289644325a15a520c5e7

Sellers:

12 keys:

https://i.imgur.com/vbZ0YtB.png

https://backpack.tf/item/6652018254

https://backpack.tf/item/6538281909

20 keys:

https://i.imgur.com/s2RJ5Pm.png

https://backpack.tf/item/6464435000

50 keys:

https://i.imgur.com/2KIc6Rt.png

https://backpack.tf/item/6544660223

Sales at:

11 (2x)

16

17

16 and 17 are outliers, given the sellers for much cheaper. Excluding 16 and 17.

Going with 11 flat.