Using last month.

Sale 1:

Sold for 14 keys

Seller: https://backpack.tf/profiles/76561198114402400#!/compare/1532822400/1532908800

Buyer: https://backpack.tf/profiles/76561198345757480#!/compare/1532822400/1532908800

https://backpack.tf/item/6475315271

X = 14

Sale 2:

Sold for $24.99 on marketplace.tf

https://marketplace.tf/items/1015;5;u3005

Can’t find item history

http://manic.tf/keyprice/?at=2018-08-22 - 24.99/1.9 = ~13.15

X = 13

Sale 3:

Sold for 13 keys

Seller: https://backpack.tf/profiles/76561198080179568#!/compare/1532563200/1532649600

Buyer: https://backpack.tf/profiles/76561198207438941#!/compare/1532304000/1532649600

https://backpack.tf/item/1856606765

X = 13

Sale 4:

Sold to a bot for 9 keys + 6.11 ref?

Seller: https://backpack.tf/profiles/76561198207438941#!/compare/1532736000/1532822400

Buyer: https://backpack.tf/profiles/76561198305676157#!/compare/1532649600/1532736000

https://backpack.tf/item/1856606765

https://i.imgur.com/Lqzn6Bx.png

Only the ref matches, but since it’s the exact amount, I doubt there’s anything wrong with the sale.

X = 9

Sale 5:

Sold for 9 keys + 6.11 ref?

Seller: https://backpack.tf/profiles/76561198310897229#!/compare/1534723200/1534809600

Buyer: https://backpack.tf/profiles/76561198305676157#!/compare/1534723200/1534809600

https://backpack.tf/item/1856606765

https://i.imgur.com/Lqzn6Bx.png

Pretty much the exact same situation as the sale above, though this time there’s a key visible in the compare link, so again, I doubt there’s anything wrong with the sale.

X = 9

Sale 6:

Sold for Orbiting Fire El Jefe?

http://www.tf2outpost.com/trade/29723294 - B/O of 13

Seller: https://backpack.tf/profiles/76561198080179568#!/compare/1532390400/1532476800

Buyer: https://backpack.tf/profiles/76561198351664215#!/compare/1532390400/1532476800

https://backpack.tf/item/1856606765

Was traded back, not sure about this sale, might be better to exclude it. Would most likely end up in the range anyways.

Untraceable bot.tf trades:

https://bot.tf/stats/item?def=1015&q=5&ef=Fountain+of+Delight&craft=1&aus=0&ks=0

https://i.imgur.com/Lqzn6Bx.png

https://backpack.tf/profiles/76561198305676157#!/compare/1532822400/1532908800

https://backpack.tf/profiles/76561198310897229#!/compare/1532822400/1532908800

https://backpack.tf/item/1856606765 - Labelled as a 12 key sale, untraceable.

https://backpack.tf/profiles/76561198335791307#!/compare/1532815200/1532901600

This compare link is the same as the 14 key sale and the 9 keys + 8 ref purchase on bot.tf. However, the item history for the 14 key sale just doesn’t appear anywhere. 14 keys do appear together in the compare, though I’m unable to determine who they came from, as a lot of the key owners vary, suggesting keys were bought on marketplace.tf or some other cash-trading website.

https://backpack.tf/item/6031989793 - As for the 9 key + 8 ref purchase, the seller is untraceable.

Sellers:

https://i.imgur.com/77Hl2yB.png

https://i.imgur.com/8uCBHFG.png

https://backpack.tf/item/1856606765 - 12 keys

https://backpack.tf/item/1848773648 - 12 keys

https://backpack.tf/item/6647768705 - 13 keys

https://backpack.tf/item/6031989793 - 14 keys

https://backpack.tf/item/1545436901 - 16 keys

https://backpack.tf/item/1712451537 - 26 keys

https://backpack.tf/item/2924463732 - 26 keys

https://backpack.tf/item/4004298732 - 26 keys

https://backpack.tf/item/1473569270 - 50 keys

Sales at:

9 (2x)

13 (2x)

14

Excluding 14, given 3 cheaper sellers, and a near-month unsold at 14, as well as the sale at 14 being a little older than some of the others.

Keeping 9, as bot.tf would suggest that bots are commonly buying for 9 keys or less: https://i.imgur.com/Lqzn6Bx.png

https://bot.tf/stats/item?def=1015&q=5&ef=Fountain+of+Delight&craft=1&aus=0&ks=0

Going with 9-13.