I recently bought this hat for a Blizzardy Storm Company Man (26.5-28keys) and received 2 keys in sweets along with it ( a Strange Stickybomb Launcher skin and a Lime Level 69 Virtual Viewfinder ). As it can be observed in other suggestions, this hat was also sold for 29 keys in the past twice and was quicksold for ~20 keys once. Overall, I would say this gives the hat a value of 28 keys.

http://m.imgur.com/YXpdDE7,3AGwoDI

Here is some proof of sale