Using last month.

Sale 1:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198311498888#!/compare/1532649600/1532736000

Buyer: https://backpack.tf/profiles/76561198059836930#!/compare/1532649600/1532736000

X = 11

Sale 2:

Sold for 9 keys + 19 ref

Seller: https://backpack.tf/profiles/76561198059836930#!/compare/1532995200/1533081600

Buyer: https://backpack.tf/profiles/76561198063481318#!/compare/1532649600/1533081600

X = 9.5

Sale 3:

Sold to a bot for 8 keys + 6 ref

Seller: https://backpack.tf/profiles/76561198073102260#!/compare/1533340800/1533427200

Buyer: https://backpack.tf/profiles/76561198312583107#!/compare/1533340800/1533427200

https://backpack.tf/item/6973033443

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

X = 8

Untraceable bot.tf sales:

https://bot.tf/stats/item?def=30615&q=5&ef=Fountain+of+Delight&craft=1&aus=0&ks=0

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

https://backpack.tf/profiles/76561198100103299#!/compare/1533340800/1533427200

https://backpack.tf/profiles/76561198334598690#!/compare/1533340800/1533427200

Labelled as an 8 key and 6 ref purchase

Sellers:

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

14 keys:

https://backpack.tf/item/6973033443

https://backpack.tf/item/6221256748

15 keys:

https://backpack.tf/item/6556346616

28 keys:

https://backpack.tf/item/6862920474

30 keys:

https://backpack.tf/item/3907027405

Sales at:

8

9.5

11

Bot.tf stats state that there have been 4 sales at ~8 keys in the last ~2 months

https://bot.tf/stats/item?def=30615&q=5&ef=Fountain+of+Delight&craft=1&aus=0&ks=0

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

Even if two of the sales are older than a month, 8 still appears to be a pretty common trading point. Keeping 8 in the range.

Going with 8-11.

https://backpack.tf/item/5584700660 - unusualified

https://backpack.tf/item/4976041213 - tradeit.gg stuff

https://marketplace.tf/items/30615;5;u3005 - Sold on marketplace.tf, can't find the item's history.

http://manic.tf/keyprice/?at=2018-08-21 - $15 / $1.9 = 7.89 keys

= 8