Using last month.

Sale 1:

Sold for $45 on Marketplace.tf

https://marketplace.tf/items/1157;5;u3010

45/1.97 = ~22.8

X = 23

Sale 2:

Sold for 21 keys

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1524268800/1524355200

Buyer: https://backpack.tf/profiles/76561198061406568#!/compare/1523059200/1524441600

Bot.tf confirms: https://i.imgur.com/47QpFTx.png

X = 21

Sale 3:

Sold for Grail Square Dance + ~10.5 keys in items

No outpost

Seller: https://backpack.tf/profiles/76561198202308457#!/compare/1523664000/1523750400

Buyer: https://backpack.tf/profiles/76561198125332511#!/compare/1523750400/1523923200

Looks pretty high, excluding.

Sellers:

20 keys:

https://i.imgur.com/Oex2HHl.png

https://backpack.tf/item/6513238121

https://backpack.tf/item/6413293099

Offers:

http://www.tf2outpost.com/trade/29640895

Sales at:

21

23

Gonna exclude 23, given the cheaper sellers. Even Chopper had one listed for 21: https://i.imgur.com/0m4DfVw.png

Using sellers at 20 as the low-end.

Going with 20-21.