Proof:

http://www.tf2outpost.com/trade/8740200 << 2 recent offers, between 3,5+ and +-4,5

http://www.tf2outpost.com/trade/8276025 << Old offers between 2- , 3-(not even quicksell price) to 3,5 , 4 and 4,5

http://www.tf2outpost.com/trade/8734905 guy offered on trade above a nuts KE, possibly got one with that. (4 buds+-)

For this PC I'm taking mainly the most recent offers on the first trade, and the more than likely sell of 4 or so buds of the KE for this hat on the third one.

Low end would be 3,5, max 4,5 within realistic terms, only basing on offers.