3 month old price. 0.9 keys = ~5 ref.

classifieds: http://backpack.tf/classifieds/search/167/600/0 - http://i.imgur.com/XrwoJKS.png

11 sellers @ 1 key, 2 sellers @ 5.44 ref

Sales below 1 key:

http://www.tf2outpost.com/trade/12259007 - 0.9 keys

http://www.tf2outpost.com/trade/12068381 - 5 ref

http://www.tf2outpost.com/trade/12044366 - 5 ref

http://www.tf2outpost.com/trade/12021875 - 5 ref

http://www.tf2outpost.com/trade/12139973 - 5 ref

http://www.tf2outpost.com/trade/12146271 - 5 ref

http://tf2tp.com/trade.php?trade=9577426 - 5 ref

http://www.tf2outpost.com/trade/12260907 - 5 ref

http://www.tf2outpost.com/trade/12253338 - 5 ref ( http://i.imgur.com/dRHxEaU.png )

http://www.tf2outpost.com/trade/12218330 - 5 ref

http://i.imgur.com/pFZ53eP.png - 5.11

http://i.imgur.com/3BjWksX.png - 5.22

http://www.tf2outpost.com/trade/12042691 - 5.33

http://www.tf2outpost.com/trade/12026341 - 5.33

http://www.tf2outpost.com/trade/12159969 - 5.33

http://www.tf2outpost.com/trade/12161123 - 5.33

http://www.tf2outpost.com/trade/12224038 - 5.33

http://bazaar.tf/trade/283923 - 5.33

http://i.imgur.com/Xdi9Yfe.png - 5.33

http://i.imgur.com/bgLnq6W.png - 5.33

http://i.imgur.com/olCCLn0.png - 5.33

http://i.imgur.com/RK2ePLc.png - 5.33

http://www.tf2outpost.com/trade/12171633 - 1 key minus 1 scrap

Outliers:

http://www.tf2outpost.com/trade/12045696 - 1 ref

http://www.tf2outpost.com/trade/12227595 - 3.66

http://www.tf2outpost.com/trade/12072830 - 4 ref

http://www.tf2outpost.com/trade/12159442 - 4 ref

http://www.tf2outpost.com/trade/12153349 - 4 ref

http://i.imgur.com/gEfTAJM.png - 4 ref

http://www.tf2outpost.com/trade/12016666 - 4.33

http://www.tf2outpost.com/trade/12258996 - 4.33

http://www.tf2outpost.com/trade/12024772 - 4.66

http://www.tf2outpost.com/trade/12124663 - 4.66 ( http://i.imgur.com/mm76AQS.png )

FAQ:

Q: Why not price it in metal? 0.9 keys is weird.

A: The price of this item has coincided with the price of keys. Many sales still occur with keys. I did not find sufficient proof to change the currency.

Q: 0.9 keys is weird.

A: type "convert 0.9 keys" into the search bar. If that is too much work, just buy it for a key.

Q: Why not just keep it at 1 key? There are always deviations in sales.

A: The deviations only occur below the price. With the large amount of sellers asking 1 key, they never sell for more. However, it is quite possible (multiple opportunities per day) to buy one for less. It is actually mentioned by some as an item you can easily buy for less than 1 key and easily sell for 1 key ( http://www.tf2confidential.com/how-to-pay-less-than-market-value-for-keys/#more-190 )

Q: but they sell at 1 key all the time.

A: which is why it is still included in the price.

Q: What are those screenshots of?

A: trade.tf keeps a cache of backpack.tf classifieds after they are closed.

I'll continue to add proof as I find it. Cheers.