10 month old price!

Sale 1:

Sold for 28 keys

http://www.tf2outpost.com/trade/26186578

Backpack of buyer:

http://backpack.tf/profiles/76561198101265558?time=1433919600 (June 10th)

http://backpack.tf/profiles/76561198101265558?time=1434178800 (June 13th)

Backpack of seller:

http://backpack.tf/profiles/76561198048008254?time=1434092400 (June 12th)

http://backpack.tf/profiles/76561198048008254?time=1434178800 (June 13th)

Sale 2:

Sold for 20 keys

http://www.tf2outpost.com/trade/26105946 (took more than a week to sell)

Backpack of buyer:

http://backpack.tf/profiles/76561198048008254?time=1434006000 (June 11th)

http://backpack.tf/profiles/76561198048008254?time=1434092400 (June 12th)

Backpack of seller:

http://backpack.tf/profiles/76561198140464191?time=1434092400 (June 12th)

http://backpack.tf/profiles/76561198140464191?time=1434178800 (June 13th)

Notes:

http://www.tf2outpost.com/trade/26014960 - B/O of 35 keys (3 weeks)

Due to Sale 2 taking more than a week to sell at 20 keys, it is safe to say that neither of the two sales provided here are outliers.

Conclusion:

Two valid sales at 20 and 28 keys, along with a B/O of 35 keys for more than 3 weeks. Taking a final range of 20-28 keys.

Feel free to support/counter my suggestion!