Bazinga

https://backpack.tf/item/5444647816

Sale 1) Sold for 15 Keys

https://backpack.tf/profiles/76561197999542517#!/compare/1497484800/1497571200

http://www.tf2outpost.com/trade/29228849 - Sold in 2 Days.

Sale 2) Sold for a Green Confetti Black Watch

https://backpack.tf/profiles/76561198153481237#!/compare/1497657600/1497744000

http://www.tf2outpost.com/trade/29232000 - Capped by the Buyout of 16 Keys

Sale 3) Sold for a Haunted Ghosts Elf Esteem (17 Keys)

https://backpack.tf/profiles/76561198087172601#!/compare/1497916800/1498003200

Sale 4) Sold + 4 Keys + ~1 Key of Items for a Duped Searing Plasma Hottie's Hoodie (24 Keys)

https://backpack.tf/profiles/76561198162694090#!/compare/1498176000/1498348800

24 - 4 - 1 = 19 Keys

Sale 5) Sold for 17 Keys + 5 Refined

https://backpack.tf/profiles/76561198119013032#!/compare/1499385600/1499472000

Rounds to 17 Keys

Sale 6) Sold + 1 Key + 2 Refined (doesn't matter here) for a Haunted Phantasm Jr Bombing Run (20 Keys)

https://backpack.tf/profiles/76561198121660651#!/compare/1499731200/1499904000 - Seller's Backpack

https://backpack.tf/profiles/76561198357315076#!/compare/1499817600/1499904000 - Buyer's Backpack

http://www.tf2outpost.com/trade/29270636 - Capped by the Buyout of 16 Keys

https://backpack.tf/item/5417654170

Sale 7) Sold for 15 Keys + 20 Refined

https://backpack.tf/profiles/76561198048940490#!/compare/1501372800/1501459200

Rounds to 16 Keys

Sale 8) Sold + 4 Keys for a Terror-Watt Whirly Warrior (27.5 Keys)

https://backpack.tf/profiles/76561198192781842#!/compare/1501459200/1501545600

http://www.tf2outpost.com/trade/29305007 - Capped by the Buyout of 19 Keys

Sale 9) Sold + 1 Key + ~1 Key of Items for a Cloud 9 FR-0 (12 Keys) + Nuts and Bolts Large Luchadore

https://backpack.tf/profiles/76561198121660651#!/compare/1502150400/1502323200

Fits in the range.

Since 15 Keys sold fairly quickly, I'll call it low.