Sale 1:

Sold for 25 keys

No outpost link available

http://backpack.tf/profiles/76561198039659875?time=1444892400&compare=1444806000

X = 25 keys

Sale 2:

Sold for Miami Nights Bullet Buzz, 4 keys and a key in items

http://www.tf2outpost.com/trade/27432623

http://backpack.tf/profiles/76561198068090261?time=1447574400&compare=1448006400

http://backpack.tf/profiles/76561198095752814?time=1447920000&compare=1447833600

X = (20 + 1)0.9 + 4

X = 23 keys

There was another sale, but it looks like it went to someone with a private backpack, so it was difficult tracking down what it had sold for.

Bullet Buzz's price is now 18-23, so it's actually:

X = (20.5 + 1)0.9 + 4

Still rounds up to 23, just worth noting