Updated price suggestion, please read whole.

The one that was listed for sale, originally 40 keys, (proof:https://imgur.com/a/ZxkKXV9)

was sold for 35 keys pure.

https://backpack.tf/profiles/76561198065520051#!/compare/1609891200/1609977600

https://backpack.tf/profiles/76561198054270952#!/compare/1609891200/1609977600

and as user Shuffle Spy helped me out, here are some more sales:

https://backpack.tf/profiles/76561198141832563#!/compare/1609459200/1609718400 - Sold for 35 Keys

https://backpack.tf/profiles/76561199043177646#!/compare/1608595200/1608681600 - Sold for 13 Keys + 13.11 Refined? (doubt thats the full price, seems like an overpay on a hat to hat trade, or addition to a paypal transaction)

https://marketplace.tf/items/tf2/538;5;u57 - Sold for $79.99 (42 Keys) and $89.99 (42 Keys)

https://backpack.tf/profiles/76561198054270952#!/compare/1608595200/1608681600 - ?

https://backpack.tf/profiles/76561198061464918#!/compare/1605571200/1605657600 - ?

also, theres a listing of a Terror-Watt Killer Exclusive on the Steam Market, for 120,75€

link :https://steamcommunity.com/market/listings/440/Unusual%20Killer%20Exclusive