The last suggestion for this hat by user Hugh Jazz Mann, forgot a key piece of evidence:

http://backpack.tf/suggestion/572df802c440450da21c3685

In addition with the Eldritch Smokey Sombrero (38) + Prof KS Aussie Wrench (35) + 40 keys

A Vanilla Gut Knife (FN) was included.

http://www.tf2outpost.com/trade/27920894 (Outpost)

Proof:

http://i.imgur.com/NKY3YM7.png (The Actual Trade)

http://csgo.backpack.tf/profiles/76561198059667511?time=1457769600

http://csgo.backpack.tf/item/4809204739

The average price for the knife is around $60 to $70 on SCM so meeting in the middle with $65 seems appropriate.

http://steamcommunity.com/market/listings/730/%E2%98%85%20Gut%20Knife

Therefore 26 keys

Before I forget, although it may be irrelevant now, the sombrero was priced at 50 keys when I made the trade. If it dosen't matter that is fine.

Forgive me if I didn't supply enough proof.