Bit outdated. Using sales from after the previous suggestion was made/accepted (6/23)

https://backpack.tf/item/7895721371

Sale 1) Bulk sale for a Scorching Flames Blighted Beak

https://backpack.tf/profiles/76561198082350482#!/compare/1562716800/1562803200

https://backpack.tf/item/6684061560

Sale 2) Bulk sale for 70 Keys

https://backpack.tf/profiles/76561198198869057#!/compare/1561766400/1561852800

Sale 3) Sold for an It's a Secret to Everybody Commando Elite (46.5 Keys)

https://backpack.tf/profiles/76561198165484704#!/compare/1561939200/1562025600

Rounds to 47 Keys

Sale 4) Bulk sale for a Scorching Flames Batter's Helmet

https://backpack.tf/profiles/76561198257064567#!/compare/1562198400/1562284800

https://backpack.tf/profiles/76561198050933686#!/compare/1562198400/1562284800

https://backpack.tf/item/7908706924

Sale 5) Sold for $99.99 when Keys were ~$1.83 each

https://backpack.tf/profiles/76561198092271981#!/compare/1565481600/1565827200

https://marketplace.tf/items/tf2/5021;6

99.99 / 1.83 = 54.64, which rounds to 55 Keys

https://backpack.tf/item/7084657793

Sale 6) Bulk sale for ? Keys

https://backpack.tf/profiles/76561198151054147#!/compare/1561766400/1561852800

https://backpack.tf/profiles/76561198055622215#!/compare/1561766400/1561852800

Sale 7) Sold + 5 Keys for a Burning Flames Dead Cone (48 Keys) + Circling TF Logo Kiss King (15 Keys)

https://backpack.tf/profiles/76561198055622215#!/compare/1562025600/1562112000

https://backpack.tf/profiles/76561198224648164#!/compare/1562112000/1562198400

48 + 15 - 5 = 58 Keys

High outlier.

Kiss King Mini

https://backpack.tf/suggestion/5d6cab076780722b761ec784 - 15 Keys

Sale 8) ?

https://backpack.tf/profiles/76561198224648164#!/compare/1563148800/1563235200

https://backpack.tf/item/7723603252

Sale 9) Gambled

https://backpack.tf/profiles/76561198062290614#!/compare/1561248000/1561334400

Sale 10) Gambled

https://backpack.tf/profiles/76561198073522694#!/compare/1561248000/1561334400

Sale 11) Gambled

https://backpack.tf/profiles/76561198834678807#!/compare/1562284800/1563062400

Sale 12) Gambled

https://backpack.tf/profiles/76561198880377562#!/compare/1563580800/1563926400

Sale 13) Gambled

https://backpack.tf/profiles/76561198078373457#!/compare/1564185600/1564358400

Sale 14) Gambled

https://backpack.tf/profiles/76561198287198615#!/compare/1564185600/1564272000

Sale 15) Sold for an Eldritch Flame Prehistoric Pullover (61 Keys) + Strange Killstreak Loose Cannon (~4 Keys)

https://backpack.tf/profiles/76561198066742724#!/compare/1564185600/1564272000

https://backpack.tf/profiles/76561198093272537#!/compare/1564272000/1564531200

https://imgur.com/a/ZegqDhj - Capped by the Buyout of 56 Keys

https://marketplace.tf/items/tf2/30422;5;u8 - Also sold for $75 when Keys were ~$1.82 each

https://marketplace.tf/items/tf2/5021;6

75 / 1.82 = 41.21, which rounds to 41 Keys

https://imgur.com/a/hF7phf3 - 9 Day Seller at 40 Keys, 10 Day Seller at 41 Keys, 6 Day Seller at 53 Keys, 2 Month Seller at 58 Keys

https://backpack.tf/item/7084657793

https://backpack.tf/item/7790346926

https://backpack.tf/item/5324852735

https://backpack.tf/item/7895721371