Using last month.

Sale 1:

Sold for $64.99 on Marketplace.tf

https://marketplace.tf/items/1172;5;u3010

64.99/1.97 = ~33

X = 33

Sale 2:

Sold for 23 keys

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1523836800/1523923200

Buyer: https://backpack.tf/profiles/76561198106291969#!/compare/1523836800/1523923200

Untraceable, but Bot.tf confirms: https://i.imgur.com/ycAeIRO.png

Buyer sold their Kabuto to the “Unusuals! :D” bot, then spent those keys on the taunt.

X = 23

Sale 3:

Sold for 18 keys

Seller: https://backpack.tf/profiles/76561198102964652#!/compare/1523836800/1523923200

Buyer: https://backpack.tf/profiles/76561198124728984#!/compare/1523923200/1524009600

X = 18

Here are the key histories so Mindacos doesn’t throw a tantrum:

https://backpack.tf/item/6784312193

https://backpack.tf/item/6784312199

https://backpack.tf/item/6784312203

https://backpack.tf/item/6784312206

https://backpack.tf/item/6784312187

https://backpack.tf/item/6784312203

https://backpack.tf/item/6784312206

https://backpack.tf/item/6784312211

https://backpack.tf/item/6784312217

https://backpack.tf/item/6784312220

https://backpack.tf/item/6784312225

https://backpack.tf/item/6784312230

https://backpack.tf/item/6784312234

https://backpack.tf/item/6784312238

https://backpack.tf/item/6784312244

https://backpack.tf/item/6784312248

https://backpack.tf/item/6784312250

https://backpack.tf/item/6784312253

https://backpack.tf/item/6784312257

https://backpack.tf/item/6784312261

https://backpack.tf/item/6784312264

https://backpack.tf/item/6784312267

https://backpack.tf/item/6784312272

Sellers:

60 keys:

https://i.imgur.com/0O4Fr3c.png

https://backpack.tf/item/5745432549

And a seller for $50 on Marketplace.tf (~25 keys):

https://marketplace.tf/items/1172;5;u3010

https://backpack.tf/item/5486945227

Sales at:

18

23

33

33 already looks like a large outlier, and the seller at $50 on Marketplace.tf confirms.

Going with 18-23.