Using last month

Sales 1, 2 & 3:

Sold for $18.5, $15 and $14.95 on Marketplace.tf

https://marketplace.tf/items/1109;5;u3010

18.5/1.97 = ~9.39

15/1.98 = ~7.58

14.95/1.98 = ~7.55

X = 9.5

X = 7.5 (2x)

Sale 4:

Sold for 6 keys (ID’s match)

Seller: https://backpack.tf/profiles/76561198066742343#!/compare/1522627200/1523059200

Buyer: https://backpack.tf/profiles/76561198803078493#!/compare/1522627200/1522800000

X = 6

Sale 5:

Quicksold for 6 keys (ID’s match)

Seller: https://backpack.tf/profiles/76561198121830870#!/compare/1523059200/1523145600

Buyer: https://backpack.tf/profiles/76561198179426894#!/compare/1523059200/1523145600

X = 6

Sale 6:

Sold for 6 keys

Seller: https://backpack.tf/profiles/76561198216954553#!/compare/1524614400/1524700800

Buyer: https://backpack.tf/profiles/76561198120984202#!/compare/1524614400/1524700800

X = 6

Sale 7:

Sold for 6 keys (ID’s match)

Seller: https://backpack.tf/profiles/76561198383491727#!/compare/1523404800/1523491200

Buyer: https://backpack.tf/profiles/76561198011707565#!/compare/1523404800/1523491200

X = 6

Sale 8:

Quicksold for 5 keys

Seller: https://backpack.tf/profiles/76561198089900002#!/compare/1523059200/1523145600

Buyer: https://backpack.tf/profiles/76561198432068902#!/compare/1523059200/1523145600

X = 5

Sale 9:

Sold for 5 keys in an auction

Seller: https://backpack.tf/profiles/76561198276924608#!/compare/1524096000/1524182400

Buyer: https://backpack.tf/profiles/76561198216954553#!/compare/1524096000/1524182400

X = 5

Sale 10:

Sold for 4 keys

Seller: https://backpack.tf/profiles/76561198803078493#!/compare/1523059200/1523145600

Buyer: https://backpack.tf/profiles/76561198168625615#!/compare/1523059200/1523145600

X = 4

Sellers:

~5.5 keys:

https://i.imgur.com/9Qc5dSW.png

https://backpack.tf/item/2943593361

7 keys:

https://i.imgur.com/M4pKBgF.png

https://backpack.tf/item/2742153642

~7.6 keys:

https://i.imgur.com/pzEp9eP.png

https://backpack.tf/item/4790179692

11 keys:

https://i.imgur.com/f4d1Alo.png

https://backpack.tf/item/2969394509

Sales at:

4

5 (2x)

6 (3x)

7.5 (2x)

9

Sellers at 7 and 7.6 keys are 2 and 3 months old respectively. Excluing 7.5 and 9.

Sales at 5 may have quicksold, but they’re somewhat common, and the seller at 5.5 supports it.

https://i.imgur.com/A9BINNl.png - Buyers at 4+ keys, excluding 4.

Going with 5-6.