5 month old price!

Sale 1:

Sold for Kill-a-Watt Tough Stuff Muffs (40) + sweets (~1) = 41 keys

http://www.tf2outpost.com/trade/27885889

http://backpack.tf/profiles/76561198102117861/#!/compare/1456473600/1457164800

http://backpack.tf/profiles/76561198071642255/#!/compare/1456387200/1457164800

Sale 2:

Sold for Vivid Plasma Chieftain's Challenge* (46.5) + Professional Australium Stickybomb Launcher (25) = 71.5 (B/O caps it at 45 keys)

http://www.tf2outpost.com/trade/28087017

http://backpack.tf/profiles/76561198154701068/#!/compare/1460764800/1461024000

Sale 3: (outlier)

Sold for Specialised Australium Minigun = ~20 keys

http://backpack.tf/profiles/76561198119937114/#!/compare/1458864000/1459036800

http://i.imgur.com/XfUnY02.png

Mini-suggestion for Vivid Plasma Chieftain's Challenge*:

http://backpack.tf/vote/id/57186172c440457b3b1b1cc3

~ Taking a value of 46.5 keys

Conclusion:

Two valid sales at 41 and 45 keys. Taking a final range of 41-45 keys.

Feel free to support/counter my suggestion!