Using last month.

Sales 1 & 2:

Sold for $37.99 and $30 on marketplace.tf

https://marketplace.tf/items/1172;5;u3004

37.99/1.98 = ~19.2

30/1.95 = ~15.4

X = 19

X = 15

Sale 3:

Sold for Whirlwind Victory Lap (Suggestion at 11) + ~4 keys in items

https://i.imgur.com/GTHhLyA.png - B/O of 15 - credit to Beaser

Seller: https://backpack.tf/profiles/76561198232041512#!/compare/1531872000/1531958400

Buyer: https://backpack.tf/profiles/76561198163238083#!/compare/1531785600/1531958400

X = 15

Sale 4:

Sold for Smoking Smokey Sombrero (See sales below) + 3 keys

Seller: https://backpack.tf/profiles/76561198215998511#!/compare/1531958400/1532044800

Buyer: https://backpack.tf/profiles/76561198112030995#!/compare/1531958400/1532044800

Sale 5:

Bulk for pure?

Seller: https://backpack.tf/profiles/76561198264117106#!/compare/1531785600/1531872000

Buyer: https://backpack.tf/profiles/76561198232041512#!/compare/1531785600/1531872000

Whirlwind Victory:

https://backpack.tf/suggestion/5b57e661cf6c752c0d1c2fe5

Smoking Smokey sales:

Sold for 5 keys + Yeti Smash Unusualifier (~5) + cheap items

Seller: https://backpack.tf/profiles/76561198215998511#!/compare/1531958400/1532044800

Buyer: https://backpack.tf/profiles/76561198053720151#!/compare/1531958400/1532044800

X = 10

Bulk for Flies Rack

Seller: https://backpack.tf/profiles/76561198053720151#!/compare/1531958400/1532044800

Buyer: https://backpack.tf/profiles/76561198055622215#!/compare/1531958400/1532044800

Sold with 1 key + cheap items for Hot Stickybomb Launcher

Seller: https://backpack.tf/profiles/76561198055622215#!/compare/1532217600/1532304000

Buyer: https://backpack.tf/profiles/76561198152860593#!/compare/1532131200/1532304000

Not enough data points for a suggestion. Better off being used on the Smokey if possible, or just not at all.

Sellers:

19 keys:

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

https://backpack.tf/item/5193077334

23 keys:

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

https://backpack.tf/item/5390716045

50 keys:

https://i.imgur.com/ud9TUZV.png

https://backpack.tf/item/6194164872

Sales at:

15 (2x)

19

Going with 15-19.