I have taking the words you said into me, and made this.

2 of 2 in the world - http://backpack.tf/stats/Unusual/Engineer's%20Cap/Tradable/Craftable/77

only 1 on tf2outpost - http://www.tf2outpost.com/search/243676159

Sold for Ghastly Ghost Jr Hotrod - http://i.imgur.com/a8gXqUH.png

Ghast Ghost Jr Hotrod is 3.2 buds with overpay - http://backpack.tf/calculator/55192409b98d88cd1c8b4715

History of the Engi Cap - http://backpack.tf/item/3120423089

Current owners backpack - http://backpack.tf/profiles/76561198006136307

29-2 x 0.9 = 26.2

26.2 keys = 3.28 buds

rounded to 3 buds

one of the previous owner sold it for 3 buds - http://gyazo.com/be23dfb1b4acf1da59863f2b87ba1449