Sale 1:

Sold for 28 keys

http://www.tf2outpost.com/trade/28528735 - B/O of 28

Seller: http://backpack.tf/profiles/76561198138213103#!/compare/1475971200/1476057600

Buyer: http://backpack.tf/profiles/76561198074394059#!/compare/1476057600/1476144000

X = 28

Sale 2:

Sold for 26 keys

http://www.tf2outpost.com/trade/28505122 - B/O of 26

Seller: http://backpack.tf/profiles/76561198108148076#!/compare/1475280000/1475366400

Buyer: http://backpack.tf/profiles/76561198138213103#!/compare/1475020800/1475366400

Sorting by time seems to be a little off here

X = 26

Sale 3:

Sold for Aces Cold Killer + 4 keys

http://www.tf2outpost.com/trade/28476929 - B/O of 25

Seller: http://backpack.tf/profiles/76561198084574795#!/compare/1473724800/1473811200

Buyer: http://backpack.tf/profiles/76561198059010618#!/compare/1473724800/1473811200

Should easily cap at the B/O, Cold Killer would have to be 20 keys or less to change the value of this sale.

X = 25 (Capped by B/O)

Sellers:

26 keys:

http://i.imgur.com/EDW6den.png - 8 days

http://backpack.tf/u/76561198108148076

http://www.tf2outpost.com/trade/28583030

29 keys:

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

http://backpack.tf/u/76561198074394059

http://www.tf2outpost.com/trade/28585150 - listed for 30 here

35 keys:

http://i.imgur.com/m5q6OWz.png - 6 months

http://backpack.tf/u/76561198032176942

http://www.tf2outpost.com/trade/28578644

Offers:

http://www.tf2outpost.com/trade/28588385

http://backpack.tf/u/76561198009163510

Sales at:

25

26

28

Going with 25-28