Using last month.

Sales 1 & 2:

Sold for $28.64 and $22.99 on Marketplace.tf

https://marketplace.tf/items/1115;5;u3009

28.64/1.97 = ~14.5

22.99/1.97 = ~11.7

X = 15

X = 12

Sale 3:

Sold for Spelled items

Seller: https://backpack.tf/profiles/76561198150340246#!/compare/1523404800/1523577600

Buyer: https://backpack.tf/profiles/76561198049737548#!/compare/1523232000/1523404800

Not worth looking into this sale further.

Sellers:

11 keys:

https://i.imgur.com/0PRbd9v.png

https://backpack.tf/item/4757480788 - Has been re-listing for a while now.

Sales at:

12

15

15 looks like an outlier. Excluding 15.

Seller at 11, using that as the low-end.

Going with 11-12.