Worlds Champion

https://backpack.tf/item/946687452

Sale 1) Sold for 15 Keys

https://backpack.tf/profiles/76561198093707809#!/compare/1526601600/1526688000

https://backpack.tf/item/610056919

Sale 2) Sold for 12 Keys

https://backpack.tf/profiles/76561198356986407#!/compare/1521244800/1521331200

https://backpack.tf/item/1505409509

Sale 3) Sold for 11 Keys

https://backpack.tf/profiles/76561198830289847#!/compare/1525824000/1525910400

Sale 4) Sold for an Orbiting Planets Bonk Helm (13 Keys)

https://backpack.tf/profiles/76561198275783474#!/compare/1525910400/1525996800

Bonk Mini

https://backpack.tf/suggestion/5b038e9dcf6c750e37264d17 - 13 Keys

Sale 5) Bulk

https://backpack.tf/profiles/76561198125905016#!/compare/1526601600/1526688000

Sale 6) Sold for 12 Keys

https://backpack.tf/profiles/76561198230964868#!/compare/1526688000/1526774400

https://backpack.tf/item/610056919

Sale 7) Sold for a Steaming Fed-Fightin' Fedora + ~3 Keys of Items

https://backpack.tf/profiles/76561198069813252#!/compare/1525392000/1525478400

There's no Mini for the Fedora

https://marketplace.tf/items/380;5;u32 - Sale at $25

Keys were $1.97 at the time, so 25 / 1.97 = 12.69, which rounds to 13 Keys

Sales at: 11, 12, 12, 13, 13, 15