8 month old price!

Sale 1:

Sold with 1 key + Sweets (2*0.9) for Ghosts Tam O' Shanter (19*0.9)

X + 1 + 1.8 = 17.1

X = 14.3 (rounding to 14 keys)

http://www.tf2outpost.com/trade/25876904

http://backpack.tf/profiles/76561198061401935?time=1432450800 (24th May)

http://backpack.tf/profiles/76561198061401935?time=1432796400 (28th May)

Sale 2: (credits to ApolloTheLion for proof of sale)

Sold for 15 keys

http://www.tf2outpost.com/trade/25808979

http://i.imgur.com/tB4ijSK.jpg

Notes:

http://www.tf2outpost.com/trade/26016873 - B/O of 13 keys

http://www.tf2outpost.com/trade/26023804 - B/O of 13 keys

http://www.tf2outpost.com/trade/26028089 - B/O of 13 keys (1 week)

Conclusion:

Two valid sales at 14 and 15 keys, along with 3 B/Os of 13 keys. Being conservative and taking a final range of 14-15 keys.

Feel free to support/counter my suggestion!