Sale 1:

Sold for 21 keys

Seller: http://backpack.tf/profiles/76561198121731353#!/compare/1480032000/1480118400

Buyer: http://backpack.tf/profiles/76561198070545837#!/compare/1479945600/1480291200

http://www.tf2outpost.com/trade/28614897 - Deleted B/O

X = 21

Sale 2:

Sold for 18 keys in ~2 days

No outpost

Seller: http://backpack.tf/profiles/76561198002757046#!/compare/1478563200/1478649600

Buyer: http://backpack.tf/profiles/76561198061665716#!/compare/1478563200/1478649600

X = 18

Sale 3:

Sold for Green Confetti Fukaamigasa

http://www.tf2outpost.com/trade/28589751 - B/O of 20

Seller: http://backpack.tf/profiles/76561198162915370#!/compare/1478390400/1478563200

Buyer: http://backpack.tf/profiles/76561198002757046#!/compare/1478476800/1478563200

Fukaa looks heavily overpriced, may be better to update it with this sale instead.

Sale 4:

Sold with Aussie Sniper Rifle for Circling Heart Berlin Brain Bowl

Seller: http://backpack.tf/profiles/76561198070545837#!/compare/1484352000/1484697600

Buyer: http://backpack.tf/profiles/76561198079990847#!/compare/1484352000/1484438400

Should be used to price the Brain

Sellers:

22 keys:

http://i.imgur.com/jdAu4LQ.png - 2 months

http://backpack.tf/u/76561198086948995

http://i.imgur.com/Ud0uZRA.png

http://backpack.tf/u/76561198058431653

http://www.tf2outpost.com/trade/28554103 - 3 months, price probably lowered at some point

Sales at:

18

21

18 sold somewhat quickly, excluding 18.

Going with 21 flat.