Sale for $15.95 on mannco on 7/30 https://mannco.store/item/440-green-confetti-unusual-snack-stack assumed to be this one since there's seemingly no items traded between transatctions:

Key prices on 7/30 were $1.66. $15.95 /$1.66 = 9.6 keys

Seemingly traded with 57 keys pure for a cloud 9 blast defense (priced at 41-48 keys with bots buying at 56 keys already and sellers at 70 keys, starting with the guy who purchased it here) https://backpack.tf/profiles/76561198314776045#!/compare/1625702400/1625788800

Realistically this would suggest it was valued at around 13 keys max (assuming that the current seller of the blast defense wants to break even), but realistically 8-10 keys since the person selling the blast defense likely wants to make a profit.

Had problems tracing literally every sale of this item on here as they seem to have been done through bots: https://backpack.tf/item/10327908486

https://backpack.tf/item/10327821286 seems to have been the one sold on mannco and https://backpack.tf/item/10346472671 seems to have been sold on the steam market (based on https://backpack.tf/profiles/76561197999751332#!/compare/1624752000/1624838400). Rest of these remain untraded or are strange.

Overall, this hat is weird to price, but would probably be 9-10 keys based off the verifiable sales.