Oh sorry MautaL, I didn't mean to counter you...

1 of 4 in existence!

Sale 1:

Sold for Cloud 9 Brigade Helm (27*0.9) + 8 keys + Sweets (~1.5) = 24.3 + 8 + 1.5 = 33.8 (rounding to 34 keys)

http://www.tf2outpost.com/trade/25981653

http://backpack.tf/profiles/76561198163347533?time=1431846000 (May 17th)

http://backpack.tf/profiles/76561198163347533?time=1431932400 (May 18th)

Sale 2: (invalid sale; outdated hat & more likely an outlier)

Sold for Terror-Watt Barnstormer

http://www.tf2outpost.com/trade/25974042

http://backpack.tf/profiles/76561198134126423?time=1431759600 (May 16th)

http://backpack.tf/profiles/76561198134126423?time=1431846000 (May 17th)

Notes:

Above are the only recent sales that have occurred. Currently none on the market, might as well price this using these sales before they become unusable.

Conclusion:

One valid sale at 34 keys. Taking a final flat price of 34 keys.

Feel free to support/counter my suggestion!