Sale 1:

Sold for $17.99 on Marketplace.tf

https://marketplace.tf/items/481;5;u56

https://backpack.tf/item/4023972175

17.99/2.02 = ~8.9

X = 9

Sale 2:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198264853358#!/compare/1510272000/1510358400

Buyer: https://backpack.tf/profiles/76561198120557893#!/compare/1510272000/1510358400

X = 9

Sale 3:

Sold for 8 keys + > 0.5 keys in ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1507075200/1507161600

Buyer: https://backpack.tf/profiles/76561198166067003#!/compare/1506988800/1507161600

One of the keys sold for some ref, which was used on the steel toe.

X = 9

Sale 4:

Sold for 8 keys + 3 ref to a bot

Seller: https://backpack.tf/profiles/76561198165914501#!/compare/1507680000/1507766400

Buyer: https://backpack.tf/profiles/76561198304138933#!/compare/1507680000/1507766400

X = 8

Sale 5:

Sold for 8 keys (and maybe other items, not sure)

Seller: https://backpack.tf/profiles/76561198112281935#!/compare/1509235200/1509321600

Buyer: https://backpack.tf/profiles/76561198076216867#!/compare/1509235200/1509321600

X = 8

Sale 6:

Sold for 8 keys (IDs match)

Seller: https://backpack.tf/profiles/76561197960833481#!/compare/1509580800/1509667200

Buyer: https://backpack.tf/profiles/76561198264853358#!/compare/1509580800/1509667200

X = 8

Sale 7:

Sold for 8 keys + 5 ref to a bot

Seller: https://backpack.tf/profiles/76561198036445124#!/compare/1503273600/1503360000

Buyer: https://backpack.tf/profiles/76561198344933095#!/compare/1503273600/1503360000

X = 8

Sales at:

8 (4x)

9 (3x)

Sales at 8 are far too common to be excluded.

Going with 8-9.