Sale 1:

Sold for 12 keys, a Pro Back Scratcher and a Pro Neon Annihilator

(12 + 3 + 2) = 17 keys

http://backpack.tf/profiles/76561197961067008?time=1429254000 (April 17th)

http://backpack.tf/profiles/76561197961067008?time=1429340400 (April 18th)

Sale 2:

Sold for an Aces High Human Cannonball

(17.5*0.9) = 15.75 rounding to 16 keys

http://backpack.tf/profiles/76561198047793562?time=1427785200 (March 31st)

http://backpack.tf/profiles/76561198047793562?time=1427871600 (April 1st)

Sale 3:

Sold for a Steaming Surgeon's Shako

(21.5*0.9) = 19.35 rounding to 19 keys

http://backpack.tf/profiles/76561198047793562?time=1427439600 (March 27th)

http://backpack.tf/profiles/76561198047793562?time=1427785200 (March 31st)

Sale 4: (outlier)

Sold for a Steaming Executioner

roughly 38 keys

http://backpack.tf/profiles/76561198064745482?time=1428649200 (April 10th)

http://backpack.tf/profiles/76561198064745482?time=1428735600 (April 11th)

Notes:

Mini for the Pro Back Scratcher

1 seller at $6.50

(6.5/2.4) = 2.70833333 rounding to 3 keys

http://steamcommunity.com/market/listings/440/Strange%20Professional%20Killstreak%20Back%20Scratcher

Mini for the Pro Neon Annihilator

1 seller at $5.75

(5.8/2.4) = 2.41666667 rounding to 2 keys

http://steamcommunity.com/market/listings/440/Strange%20Professional%20Killstreak%20Neon%20Annihilator

Conclusion:

Sales at 16, 17 and 19 with an outlier at 38 keys, taking the final range at 16 to 19 keys.