Little proof out there for this, only one page on outpost and bazaar currently. Not many people have them around anymore and nobody really wants/needs them, which is why it's lowering price quite rapidly.

Proof

Classifieds at the time of this suggestion

1x at 12 keys

3x at 12.37 keys

1x at 12.49 keys

http://backpack.tf/classifieds/search/id/52c8e6694dd7b8e9718b456b

Seller at 12.5 keys

http://www.tf2outpost.com/trade/19988675

Seller at 12 keys + 3 ref

http://www.tf2outpost.com/trade/19918835

Sellers at 12 keys

http://www.tf2outpost.com/trade/19977598

http://www.tf2outpost.com/trade/20006837 (Stated Quicksell)

Unsold at 13 keys (Price updated 8 days ago)

http://www.tf2outpost.com/trade/19960132 (2 days)

http://www.tf2outpost.com/trade/19927189 (4 days)

http://www.tf2outpost.com/trade/19926458 (4 days)

http://www.tf2outpost.com/trade/19801706 (1 week)

http://bazaar.tf/trade/813149 (1 week)