Sale 1:

Sold for 13 keys + ~3 keys in items (skin is a spec. ks factory new dragon slayer)

http://www.tf2outpost.com/trade/29442855 - B/O of 16

Seller: https://backpack.tf/profiles/76561198092535372#!/compare/1510876800/1511049600

Buyer: https://backpack.tf/profiles/76561197978674870#!/compare/1510963200/1511049600

X = 16

Sale 2:

Sold for Pro. KS Aussie Tomislav (16)

http://www.tf2outpost.com/trade/29449091 - B/O of 16

Seller: https://backpack.tf/profiles/76561198438004261#!/compare/1511049600/1511136000

Buyer: https://backpack.tf/profiles/76561198041566332#!/compare/1511049600/1511136000

X = 16

Sale 3:

Sold for $29.99 on Marketplace.tf

https://marketplace.tf/items/1119;5;u3016

https://backpack.tf/item/2695433970

29.99/1.98 = ~15.1

X = 15

Sale 4:

Bulk for Screaming Tiger Flip

Seller: https://backpack.tf/profiles/76561198059341397#!/compare/1509148800/1509408000

Buyer: https://backpack.tf/profiles/76561198052736868#!/compare/1509148800/1509235200

Sale 5:

Bulk for Ooze One Man Army

Seller: https://backpack.tf/profiles/76561198052736868#!/compare/1509667200/1509840000

Buyer: https://backpack.tf/profiles/76561198066711282#!/compare/1508025600/1509840000

Sale 6:

Sold with ~1 key in items for Infernal Flames Bad Pipes

Seller: https://backpack.tf/profiles/76561198039768758#!/compare/1511481600/1511568000

Buyer: https://backpack.tf/profiles/76561198033378852#!/compare/1511481600/1511568000

Given how stable this taunt appears to be, it might actually be usable for the Bad Pipes, assuming there are no better sales on them.

Sellers:

16 keys:

https://i.imgur.com/ykP3UHG.png - young

17 keys:

https://i.imgur.com/srC5anI.png - 8 days

https://i.imgur.com/ZA74MV6.png - 2 days

18 keys:

https://i.imgur.com/OlQzcw8.png - young

Offers:

http://www.tf2outpost.com/trade/29433886

Sales at:

15

16 (2x)

Going with 15-16.