Sale 1:

Sold with a Prof. Collector’s Flaregun + sweets for a Dead Presidents Le Party Phantom

(32.5*0.9) - 10 - 3 = 16.25 rounding to 16 keys

http://backpack.tf/profiles/76561198123251772?time=1433660400 (June 7th)

http://backpack.tf/profiles/76561198123251772?time=1433746800 (June 8th)

Sale 2:

Sold for 8 keys and 158 ref (keys were 19)

(158/19) + 8 = 16.31578947 rounding to 16 keys

Had many different outpost trades, sold on this one http://www.tf2outpost.com/trade/26031251

http://backpack.tf/profiles/76561198144307412?time=1430290800 (April 29th)

http://backpack.tf/profiles/76561198144307412?time=1432710000 (May 27th)

Sale 3:

Sold for a Circling Heart Pugilist's Protector or 15 keys pure?

15 keys

http://backpack.tf/profiles/76561198115974835?time=1432450800 (May 24th)

http://backpack.tf/profiles/76561198115974835?time=1432537200 (May 25th)

Notes:

Mini for the Prof. Collector’s Flaregun

http://steamcommunity.com/market/listings/440/Collector%27s%20Professional%20Killstreak%20Flare%20Gun

Sale at 26 and a seller at 25

(25/2.4) = 10.4166667 rounding to 10 keys

Mini for the Prof. S.F. Huntsman

http://steamcommunity.com/market/listings/440/Strange%20Professional%20Killstreak%20Festive%20Huntsman

2 sellers at 27.4

(27.4/2.4) = 11.4166667 rounding to 11 keys

The one in question has 5.7 keys in strange parts (5.7/2) + 11 = 13.85 rounding to 14 keys

Mini for the Circling Heart Pugilist's Protector

Sold for an australium ambassador ~15 keys

http://backpack.tf/profiles/76561198067289381?time=1434438000 (June 16th)

http://backpack.tf/profiles/76561198067289381?time=1434524400 (June 17th)

Conclusion:

2 sales at 15 and 2 at 16, the sale on the other suggestion is an outlier, also SCM sales apparently support the drop. Taking the final range at 15 to 16 keys.