Not many sellers on outpost, about 2 pages and yeah, still nobody has success with selling it for 1 key, so we'll go down. But please notice that I don't have such a huge proof, because there are more 1 key unsolds then sellers for a lower value.

Sellers for 5-5.33 ref:

http://www.tf2outpost.com/trade/12886851 - 5.11 ref.

http://www.tf2outpost.com/trade/12841024 - 5.33 ref.

http://www.tf2outpost.com/trade/12892370 - 5.33 ref.

http://www.tf2outpost.com/trade/12837519 - 5.33 ref.

http://www.tf2outpost.com/trade/12884629 - 5 ref.

http://www.tf2outpost.com/trade/12902557 - 5 ref.

http://backpack.tf/stats/30084/6/0 - 1 seller for 5 ref.

Unsold for 1 key

http://www.tf2outpost.com/trade/12807864 - 3 days

http://www.tf2outpost.com/trade/12865833 - 1 day

http://www.tf2outpost.com/trade/12742857 - 1 day

http://www.tf2outpost.com/trade/12831834 - 2 days

http://www.tf2outpost.com/trade/12866699 - 1 day

http://www.tf2outpost.com/trade/12831438 - 2 days

http://www.tf2outpost.com/trade/12881264 - 21 hours

http://www.tf2outpost.com/trade/12864366 - 1 day

http://www.tf2outpost.com/trade/12867131 - 1 day

http://www.tf2outpost.com/trade/12835089 - 2 days

http://www.tf2outpost.com/trade/12876992 - 1 day

http://www.tf2outpost.com/trade/12854639 - 1 day

http://bazaar.tf/trade/312206 - 2 days

http://tf2tp.com/trade.php?trade=9637891 - 2 days

If you will tell me that my proof is not enough, I would agree. but there are too many 1 key-unsolds to keep that price alive. And the range 5.33-5.66 is not possible, because nobody is able to sell it for one pure key or 5.66 ref.