Not much proof available, as most people are level swapping or selling in sets, and buyers just want certain levels.

http://www.tf2outpost.com/trade/9365834 Offer of 4 ref, 1 key, 1 key + 1 ref, and a 3 ref offerer who came to me yesterday rudely and offered 3.77

http://tinypic.com/r/2dt99bd/4 The two I had, one sold for 1 key, the other for 1 key + 1 ref

http://www.tf2outpost.com/trade/10218011 My second trade

http://www.tf2outpost.com/trade/10041027 My first trade

http://www.tf2outpost.com/trade/10079705 Buying for 3.33 ref, ended up getting it off me for a key

http://www.tf2outpost.com/trade/10131125 Buying for 3 ref, unsuccessful

Also note there IS one on bazaar.tf for 3 ref, but it hasn't been bumped in 2 months. Yes, 2 months.

Also note that 5 months ago key prices were ~ 3 ref.

I'm still not really sure about this myself. So please let me know what you think.