Using last month.

Sale 1:

Sold for 11 keys + ~2 keys in metal

Seller: https://backpack.tf/profiles/76561198236425506#!/compare/1539302400/1539475200

Buyer: https://backpack.tf/profiles/76561198091786633#!/compare/1539475200/1539561600

https://backpack.tf/item/6335877906

X = 13

Sales 2-8:

Sold for $17.45, $17.36, $13.99 (2x), $13.98, & $8.66 (2x) on marketplace.tf

https://marketplace.tf/items/30843;5;u3003

https://manic.tf/keyprice/?at=2018-10-04 - $17.45 / $1.8 = 9.69 keys

https://manic.tf/keyprice/?at=2018-10-14 - $17.36 / $1.77 = 9.81 keys

https://manic.tf/keyprice/?at=2018-09-20 - $13.99 / $1.84 = 7.60 keys

https://manic.tf/keyprice/?at=2018-10-01 - $13.99 / $1.83 = 7.64 keys

https://manic.tf/keyprice/?at=2018-09-30 - $13.98 / $1.84 = 7.60 keys

https://manic.tf/keyprice/?at=2018-10-03 - $8.66 / $1.8 = 4.81 keys

https://manic.tf/keyprice/?at=2018-10-09 - $8.66 / $1.79 = 4.84 keys

X = 10

X = 9.5

X = 7.5 (3x)

X = 5 (2x)

Sale 9:

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198104979324#!/compare/1538092800/1538179200

Buyer: https://backpack.tf/profiles/76561198047844129#!/compare/1538092800/1538352000

https://backpack.tf/item/6963937554

X = 10

Sale 10:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198334598690#!/compare/1536969600/1537056000

Buyer: https://backpack.tf/profiles/76561198024332838#!/compare/1536969600/1537056000

https://backpack.tf/item/7134655094

https://i.imgur.com/1jSd9Xe.png

X = 9

Sale 11:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198334598690#!/compare/1537747200/1537833600

Buyer: https://backpack.tf/profiles/76561198107393726#!/compare/1537747200/1537833600

https://backpack.tf/item/6262943726

https://i.imgur.com/1jSd9Xe.png

X = 9

Other Sales:

https://backpack.tf/item/6963937554 (#1)

https://backpack.tf/item/6670439306 (#3 & #4)

https://backpack.tf/item/7067784138 - scrap, marketplace, and not unusual at the time.

https://backpack.tf/item/7057873826 - tradeit and tf2mart

https://backpack.tf/item/6626256033 - marketplace

https://backpack.tf/item/6337608866 - supercargo bot thing

https://backpack.tf/item/6294745783 - marketplace

https://backpack.tf/item/6265670180 - marketplace and not unusual at the time

#1

Untraceable

https://backpack.tf/profiles/76561198080179568#!/compare/1537228800/1537315200

https://backpack.tf/profiles/76561198119601623#!/compare/1537315200/1537401600

It’s likely that it sold on marketplace

#2

Bulk for Spellbound Executioner

https://backpack.tf/profiles/76561198119601623#!/compare/1537574400/1537747200

https://backpack.tf/profiles/76561198104979324#!/compare/1537660800/1537747200

#3

Sold for 6 keys + some ref?

https://backpack.tf/profiles/76561198311498888#!/compare/1538352000/1538438400

https://backpack.tf/profiles/76561198231709968#!/compare/1538352000/1538438400

I’m pretty sure more was involved, but was spent later that day

#4

Probably gambled

https://backpack.tf/profiles/76561198231709968#!/compare/1538352000/1538438400

https://backpack.tf/profiles/76561198197481041#!/compare/1538352000/1538524800

6 key purchase listed on bot.tf came from a scammer

https://i.imgur.com/1jSd9Xe.png

https://backpack.tf/item/6262943726

Sellers:

https://i.imgur.com/ier5GfM.png

https://backpack.tf/item/7123741073 - 13 keys

https://backpack.tf/item/6355039314 - 18 keys

https://backpack.tf/item/6270164824 - 30 keys

Also worth noting 2 on marketplace for around 10 keys worth:

https://marketplace.tf/items/30843;5;u3003

https://backpack.tf/item/6543742245

https://backpack.tf/item/6294745783

Sales at:

5 (2x)

7.5 (3x)

9 (2x)

9.5

10 (2x)

13

13 is an outlier, as supported by mp.tf sellers. Excluding 13.

5 is countered by buyers, excluding 5. https://i.imgur.com/9KBHU8N.png

Going with 7.5-10.