Sale 1: (credits to T.w² // Bowie)

Sold for 30 keys

http://i.imgur.com/LgvTT9X.png

Sale 2: (sale was used to price the Skullcracker)

Involved in a bulk deal for '72 Skullcracker

http://www.tf2outpost.com/trade/26331917

http://backpack.tf/profiles/76561198031371895?time=1435647600 (June 30th)

http://backpack.tf/profiles/76561198031371895?time=1435756246 (July 1st)

http://i.imgur.com/a8Emwps.png

Sale 3: (credits to ᴾᴾᴹ Shuffle Spy)

Sold for 30 keys

http://backpack.tf/profiles/76561197973029544?time=1432018800 (May 19th)

http://backpack.tf/profiles/76561197973029544?time=1432191600 (May 21st)

Sale 4:

Sold for 35 keys

http://www.tf2outpost.com/trade/26383753 | http://www.tf2outpost.com/trade/26349695

http://backpack.tf/id/xyneron?time=1436166000 (July 6th)

http://backpack.tf/id/xyneron?time=1436282710 (July 7th)

http://i.imgur.com/Jln3T8I.png

Notes:

The range from this suggestion would result in more than a 5% difference from the previous suggestion.

http://www.tf2outpost.com/trade/26349695 - received an offer of 36 keys but his keys were untradable.

http://www.tf2outpost.com/trade/25869258 - B/O of 35 keys (~2 months, been frequently opening/closing his trades)

Conclusion:

Valid sales at 30, 30 and 35 keys. Taking a final range of 30-35 keys.

Feel free to support/counter my suggestion!