I don't know why the high value is 14 keys when people are selling for lower :/

link 1: http://www.tf2outpost.com/trade/9560337 - 13 keys

link 2: http://www.tf2outpost.com/trade/9541508 - bills hat and 1 key

link 3: http://www.tf2outpost.com/trade/9563438 - 13 keys

link 4: http://www.tf2outpost.com/trade/9569891 - 13 keys

link 5: http://www.tf2outpost.com/trade/9570406 - 12 keys

link 6: http://www.tf2outpost.com/trade/9563556 - sold for 11 keys

link 7: http://www.tf2outpost.com/trade/9564866 - sold for 12 and 1 key in metal

link 8: http://www.tf2outpost.com/trade/9480493 - someone wanted to sell for 11 keys

link 9: http://www.tf2outpost.com/trade/9554702 - got offered 10 and 12

link 10: http://www.tf2outpost.com/trade/9507458 - got offered 11 keys

link 11: http://backpack.tf/classifieds/search/invis - 2 people selling for 12

you can find people that want to do 11 keys but they're kinda hard to find :/

btw this is my first suggestion so please tell me if I did anything wrong :)