https://backpack.tf/profiles/76561198337734785#!/compare/1530576000/1530662400

https://backpack.tf/profiles/76561198081631601#!/compare/1530403200/1530748800

Sold for ~12 keys. Matches on seller's side.

https://backpack.tf/profiles/76561198081631601#!/compare/1526688000/1526774400

https://backpack.tf/profiles/76561198337734785#!/compare/1526688000/1526774400

Sold for ~9 keys. Low. Supports low end though.

https://marketplace.tf/items/30863;5;u30

Sold for ~15 keys.

https://backpack.tf/profiles/76561198147318341#!/compare/1530316800/1530403200

https://backpack.tf/profiles/76561198074924284#!/compare/1530230400/1530316800

Bulk.

Worth noting that the person who paid 14 keys later resold for 9 keys + ref. Either way the sale still supports the 15 key high end.

https://backpack.tf/profiles/76561198147318341#!/compare/1529798400/1530316800

https://backpack.tf/item/6485185442 - assuming this is the item history that matches the marketplace sale, it actually sold relatively quick as well at 15 keys.

With sales here at 11 / ~12 / 14 / 15 I'd go with 11-15 keys here.