since no one else is making a counter-suggestion, I guess I will

classifieds

http://i.imgur.com/rUZIFkY.png - http://backpack.tf/classifieds/search/id/521f093eba25360a3f8b457e

4 @ 3 keys (one of which is selling for 2.5 elsewhere), 2 @ 4 keys, 3 higher

sold

http://www.tf2outpost.com/trade/14124345 - sold 2 keys

http://www.trade.tf/trade/509892028 - sold 2 keys

http://www.trade.tf/trade/509873654 - sold 2 keys

http://www.tf2outpost.com/trade/14119775 - sold 2.5 keys ( http://www.trade.tf/trade/509923220 )

http://www.tf2outpost.com/trade/14100580 - seems to have been able to sell a level 100 one for 4.5 ( http://www.trade.tf/trade/509893504 )

selling

http://bazaar.tf/trade/398692 - 2.5 keys (from classifieds) [worth noting that this is the person who made the 3-5 suggestion]

http://www.tf2outpost.com/trade/14114731 - 3 keys

http://www.tf2outpost.com/trade/14100327 - "3 Keys for the strange Teddy, 4 keys B/O" idk

http://tf2deal.ru/trade.php?id=44474 - 4 keys

http://www.tf2outpost.com/trade/14108364 - 4 keys

http://www.tf2outpost.com/trade/14087661 - 5, no offers

http://www.tf2outpost.com/trade/14086077 - 4-5

http://www.tf2outpost.com/trade/14082422 - bills

http://www.tf2outpost.com/trade/14080779 - 2 unsold at 4 keys for a day and a half

they don't really seem to be selling at 3+ keys, but the 2 key sellers are selling nearly immediately. given the number available at 3 keys, the quick sales at 2, and the one unsold at 2.5, I think this is the most appropriate range.

criticism is welcome.