Sale 1:

Sold for $31.49 on Marketplace.tf

https://marketplace.tf/items/340;5;u36

https://backpack.tf/item/1597712370

31.49/1.95 = ~16.1

X = 16

Sale 2:

Sold for Aces High Brain Bucket (12.5) + ~2.5 in pure/items

http://www.tf2outpost.com/trade/29353016 - B/O of 15

Seller: https://backpack.tf/profiles/76561198127973248#!/compare/1504828800/1504915200

Buyer: https://backpack.tf/profiles/76561198147927648#!/compare/1504569600/1504915200

X = 15

Sale 3:

Sold with ~2 keys in items for some unusual smg (16)

No outpost

Seller: https://backpack.tf/profiles/76561198353418874#!/compare/1501113600/1501286400

Buyer: https://backpack.tf/profiles/76561198376090319#!/compare/1501200000/1501286400

X = 14

Sale 4:

Sold for 12 keys + > 0.5 keys in items

http://www.tf2outpost.com/trade/29266399 - B/O of 13

Seller: https://backpack.tf/profiles/76561198072491439#!/compare/1500076800/1500163200

Buyer: https://backpack.tf/profiles/76561198207956136#!/compare/1500076800/1500163200

X = 13

Sale 5:

Sold for Nuts n Bolts Tyrantium (In-date at 12, seller at 10, taking 11) + 2 keys + ~1 key in metal

http://www.tf2outpost.com/trade/29385316 - B/O of 13

Seller: https://backpack.tf/profiles/76561198126724119#!/compare/1507075200/1507161600

Buyer: https://backpack.tf/profiles/76561198342079365#!/compare/1507075200/1507161600

X = 13 (Capped by B/O)

Sale 6:

Sold with 1 key + some ref for Bubbling Towering Pillar of Hats

http://www.tf2outpost.com/trade/29332892 - B/O of 12

Seller: https://backpack.tf/profiles/76561198020067714#!/compare/1503273600/1503792000

Buyer: https://backpack.tf/profiles/76561198257431796#!/compare/1503705600/1503792000

Will easily cap at the B/O. Buyers for any effect on the tower for 13+.

X = 12 (Capped by B/O)

Sale 7:

Quicksold for 11 keys

http://www.tf2outpost.com/trade/29384351 - B/O of 13

Seller: https://backpack.tf/profiles/76561198149345069#!/compare/1506729600/1506816000

Buyer: https://backpack.tf/profiles/76561198126724119#!/compare/1506729600/1506816000

X = 11

Sale 8:

Sold for 11 keys

No outpost

Seller: https://backpack.tf/profiles/76561198093743250#!/compare/1506211200/1506297600

Buyer: https://backpack.tf/profiles/76561198114604882#!/compare/1505692800/1506297600

X = 11

Sale 9:

Sold with ~1 key in pure/items for some other unusual smg

Seller: https://backpack.tf/profiles/76561198376090319#!/compare/1503619200/1503705600

Buyer: https://backpack.tf/profiles/76561198304001962#!/compare/1503619200/1503705600

Should be used on that smg.

Sale 10:

Bulk for Strange Normal Flamethrower

Seller: https://backpack.tf/profiles/76561198142753781#!/compare/1506902400/1507075200

Buyer: https://backpack.tf/profiles/76561198324455724#!/compare/1506988800/1507075200

Sale 11:

Sold for DBD Cyborg Stunt Helmet

Seller: https://backpack.tf/profiles/76561198207956136#!/compare/1500163200/1500681600

Buyer: https://backpack.tf/profiles/76561198249553546#!/compare/1500422400/1500508800

Can be used on the Stunt Helmet.

Sale 12:

Sold with ~2 keys in items for Strange Neutron Star Wing Mann

Seller: https://backpack.tf/profiles/76561198249553546#!/compare/1500508800/1500595200

Buyer: https://backpack.tf/profiles/76561198153324766#!/compare/1500508800/1500595200

Should be used on the Wing Mann.

Sale 13:

Sold with a key for Cloud 9 Airdog

Seller: https://backpack.tf/profiles/76561198153324766#!/compare/1500768000/1500940800

Buyer: https://backpack.tf/profiles/76561198259452676#!/compare/1500940800/1501027200

Should be used on the Airdog.

Sale 14:

Sold for a bunch of items

Seller: https://backpack.tf/profiles/76561198159386980#!/compare/1504828800/1504915200

Buyer: https://backpack.tf/profiles/76561198333702358#!/compare/1504915200/1505001600

Sellers:

13 keys:

https://i.imgur.com/6F8QyQB.png - 17 days

~13.45 keys:

https://i.imgur.com/6F8QyQB.png - 3 months

14 keys:

https://i.imgur.com/lJ5xsPD.png

https://i.imgur.com/WKY43iw.jpg

16 keys:

https://i.imgur.com/oD7zaOA.png

Sales at:

11 (2x)

12

13 (2x)

14

15

16

Buyers at 11+ keys:

https://i.imgur.com/n9NtSOu.png

Excluding 11.

3-month-old seller at 13.5. Excluding sales higher than 13.5.

Going with 12-13.