When there are more than 10 for sale you know it's probably overpriced.

Sale #1:

http://www.tf2outpost.com/trade/27138290

Sold for Nuts Lucha (13.5) + 3.5 keys

http://backpack.tf/profiles/76561198115721685?time=1443510000 (September 29th)

http://backpack.tf/profiles/76561198115721685?time=1443596400 (September 30th)

(13.5 * 0.9) + 3.5 = 15.65; rounded to 16 keys

Sale #2:

http://www.tf2outpost.com/trade/27068603

Sold for 14 keys in approximately 5 days

http://backpack.tf/profiles/76561198219964760?time=1443078000 (September 24th)

http://backpack.tf/profiles/76561198219964760?time=1443164400 (September 25th)

Sale #3:

http://www.tf2outpost.com/trade/27061940

Sold for a Strange Professional Killstreak Rocket Launcher (~12 keys)

http://backpack.tf/profiles/76561198121560008?time=1442559600 (September 18th)

http://backpack.tf/profiles/76561198121560008?time=1442646000 (September 19th)

This is probably a little low, it was up for 1 day.

Sale #4:

http://www.tf2outpost.com/trade/26448876

Appears to have sold for a Strange Professional Killstreak Rocket Launcher (~12 keys) + Vintage Glengarry (0.25 keys)

http://backpack.tf/profiles/76561198134999467?time=1438671600 (August 4th)

http://backpack.tf/profiles/76561198134999467?time=1438844400 (August 6th)

Sale #5:

http://www.tf2outpost.com/trade/26409951

Sold for a Planets Tosslecap (16.5*)

http://backpack.tf/profiles/76561198027882328?time=1437807600 (July 25th)

http://backpack.tf/profiles/76561198027882328?time=1437980400 (July 27th)

16.5 * 0.9 = 15 keys

Planets Tosslecap:

http://www.tf2outpost.com/trade/27001840

Sold with ~4 keys in items for a Planets Yellow Belt (20.5)

http://backpack.tf/profiles/76561198056531196?time=1442127600 (September 13th)

http://backpack.tf/profiles/76561198056531196?time=1442214000 (September 14th)

20.5 - 4 = 16.5 keys

Outpost Sellers:

http://www.tf2outpost.com/trade/27246003 16 keys - first listed here: http://www.tf2outpost.com/trade/27215769

http://www.tf2outpost.com/trade/27111371 Unsold at 19 keys for 3 weeks

Classifieds:

http://i.imgur.com/Ynx8jGo.png

Sales are 12, 12, 14, 15, and 16. Gonna go with 14-16 since 12 quicksold.