Using last month.

Sale 1:

Sold for $9.99 on marketplace.tf (29 aug)

https://marketplace.tf/items/1116;5;u3001

https://backpack.tf/item/4362157557

http://www.tf2outpost.com/trade/29753774 - B/O of 5

http://manic.tf/keyprice/?at=2018-08-29 - $9.99 / $1.88 = 5.31 keys

X = 5 (Capped by B/O)

Sale 2:

Sold for 4 keys

Seller: https://backpack.tf/profiles/76561198140565712#!/compare/1534982400/1535068800

Buyer: https://backpack.tf/profiles/76561198849646740#!/compare/1535068800/1535155200

https://backpack.tf/item/3073929793

X = 4

Sale 3:

Sold for 4 keys

Seller: https://backpack.tf/profiles/76561198066223704#!/compare/1534636800/1534809600

Buyer: https://backpack.tf/profiles/76561198065842276#!/compare/1534636800/1535068800

https://backpack.tf/item/2786265678

X = 4

Sale 4:

Sold to a bot for 3 keys + ~0.5 keys in metal

Seller: https://backpack.tf/profiles/76561198068299933#!/compare/1535587200/1535673600

Buyer: https://backpack.tf/profiles/76561198363395191#!/compare/1535587200/1535673600

https://backpack.tf/item/4362157557

X = 3.5

Other Sales:

https://backpack.tf/item/7116836626 (#1)

https://backpack.tf/item/4362157557 (#2)

https://backpack.tf/item/3126600634 (#3 & #4)

https://backpack.tf/item/3085206712 (#5)

https://backpack.tf/item/3073929793 (#6)

https://backpack.tf/item/3465298013 - mannco.trade stuff

https://backpack.tf/item/5781947694 - tf2mart stuff/unusual at the time

https://backpack.tf/item/6035486797 - Tradeit stuff

https://backpack.tf/item/4609925292 - Scammers

https://backpack.tf/item/2768435685 - Scammers

#1

Untraceable

https://backpack.tf/profiles/76561198095812833#!/compare/1536969600/1537056000

https://backpack.tf/profiles/76561198179663741#!/compare/1536969600/1537056000

#2

Sold to a bot for 3 keys + small amount of ref?

https://backpack.tf/profiles/76561198376331369#!/compare/1534896000/1534982400

https://backpack.tf/profiles/76561198155110999#!/compare/1534896000/1534982400

I’m convinced more ref was involved, but spent on something else later that day. Marking the sale as untraceable.

#3

Sold for Mega Strike Bucking Bronco

https://backpack.tf/profiles/76561198102708483#!/compare/1535414400/1536364800

https://backpack.tf/profiles/76561198115556074#!/compare/1536105600/1536192000

Would be better to use on the Bronco, or not use at all. (No B/O)

#4

Untraceable

https://backpack.tf/profiles/76561198115556074#!/compare/1536192000/1537228800

https://backpack.tf/profiles/76561198334598690#!/compare/1536105600/1536192000

https://i.imgur.com/Uapamud.png - labelled as 3 keys + 18.11 ref

#5

Untraceable

https://backpack.tf/profiles/76561198395173649#!/compare/1536364800/1536451200

https://backpack.tf/profiles/76561198071059789#!/compare/1536278400/1536451200

#6

Bulk for pure

https://backpack.tf/profiles/76561198049599641#!/compare/1534982400/1535068800

https://backpack.tf/profiles/76561198140565712#!/compare/1534982400/1535068800

Sellers:

https://i.imgur.com/YR3z46d.png

https://backpack.tf/item/4362157557 - ~4.28 keys

https://backpack.tf/item/3097931611 - ~4.68 keys

https://backpack.tf/item/3126600634 - ~4.68 keys

https://backpack.tf/item/2877655899 - ~4.73 keys

https://backpack.tf/item/6454963630 - 6 keys

https://backpack.tf/item/3694186518 - 7 keys

Sales at:

3.5

4 (2x)

5

Even with the B/O cap, the sale at 5 is too high, given the unsolds at less than 5 keys. Excluding 5.

Going with 3.5-4.