Using last month.

Sale 1:

Sold for Grail Rock, Paper, Scissors (12.5)

http://www.tf2outpost.com/trade/29698620 - No visible B/O

Seller: https://backpack.tf/profiles/76561198124728984#!/compare/1534464000/1534550400

Buyer: https://backpack.tf/profiles/76561198110267417#!/compare/1534377600/1534550400

https://backpack.tf/item/4823906087

X = 13

Sales 2 & 3:

Sold for $23.99 and $20.65 on marketplace.tf

https://marketplace.tf/items/1107;5;u3005

https://backpack.tf/item/6272097802 - No outpost

http://manic.tf/keyprice/?at=2018-07-28 - 23.99/1.99 = ~12.06

https://marketplace.tf/items/5021;6/Mann%20Co.%20Supply%20Crate%20Key - 20.65/1.89 = ~10.93 (manic.tf doesn’t work on 18th August)

X = 12

X = 11

Sale 4:

Sold for 7 keys and 16 ref to a bot

Seller: https://backpack.tf/profiles/76561198135348194#!/compare/1529539200/1533168000

Buyer: https://backpack.tf/profiles/76561198839307425#!/compare/1532988000/1533074400

https://backpack.tf/item/6103242563

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

Keys don’t appear in the seller’s end, but all the ref does. I highly doubt bot.tf is incorrect here.

X = 7.5

Sale 5:

Bulk for Cloudy Moon Cotton Head

Seller: https://backpack.tf/profiles/76561198144628116#!/compare/1533168000/1534636800

Buyer: https://backpack.tf/profiles/76561198411930230#!/compare/1534550400/1534636800

https://backpack.tf/item/3206576263

Sale 6:

Bulk for Burning Nunhood

Seller: https://backpack.tf/profiles/76561198411930230#!/compare/1534636800/1534723200

Buyer: https://backpack.tf/profiles/76561198296076323#!/compare/1534636800/1534723200

https://backpack.tf/item/3206576263

Untraceable bot.tf trades:

https://bot.tf/stats/item?def=1107&q=5&ef=Fountain+of+Delight&craft=1&aus=0&ks=0

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

https://backpack.tf/profiles/76561198188835167#!/compare/1534032000/1534204800

https://backpack.tf/profiles/76561198839307425#!/compare/1534204800/1534291200

https://backpack.tf/item/3707243587 - Labelled as a 7 key and 25 ref purchase.

Sellers:

https://i.imgur.com/g5Dr2Z1.png

https://i.imgur.com/ZpM2y2H.png

https://backpack.tf/item/3707243587 - ~14.75 keys

https://backpack.tf/item/6103242563 - ~14.75 keys

https://backpack.tf/item/4823906087 - 15 keys

https://backpack.tf/item/2896727109 - 20 keys

https://backpack.tf/item/2712151582 - 25 keys

https://backpack.tf/item/3376555378 - 27 keys

https://backpack.tf/item/2705969810 - 40 keys

https://backpack.tf/item/2818017541 - 45 keys

Sales at:

7.5

11

12

13

7.5 doesn’t appear to be common, so I’ll exclude it.

Going with 11-13.