Sale 1 :

Before : http://backpack.tf/profiles/76561198065769283?time=1427612400

After : http://backpack.tf/profiles/76561198065769283?time=1427871600

Sold for 11 keys.

Sale 2 :

Before : http://backpack.tf/profiles/76561198160094354?time=1428908400

After : http://backpack.tf/profiles/76561198160094354?time=1429254000

Sold for a pro ks festive ambassador + 2 keys in items : 11 keys

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

Sales at :

21$

25$

23$

23$

23$

24$

Average of : 23$ : 23 / 2.45 ( price of keys at the time ) = 9 keys

Sale 3 :

Before : http://backpack.tf/profiles/76561198104085405?time=1428303600

After : http://backpack.tf/profiles/76561198104085405?time=1428390000

Sold for a Kill-a-Watt Cadaver's Cranium : 11 * 0.9 = 10 keys

Taking the range of 10 to 11 keys.

Supportive criticism welcome.