A quick search on outpost gave the following proof that 13-15 keys is a good price range:

[url]http://www.tf2outpost.com/trade/7137578[/url] One offer for 15 keys, one offer for TS Bills and 4 keys (15 keys total)

[url]http://www.tf2outpost.com/trade/6125563[/url] BMOC and a key (13 keys total)

[url]http://www.tf2outpost.com/trade/8354620[/url] One offer of Bubbling Magnificent Mongolian (13-14 keys)

[url]http://www.tf2outpost.com/trade/8304595[/url] One offer of a Spine Chilling Skull and 3 keys (15 keys total)

[url]http://www.tf2outpost.com/trade/8620831[/url] Offer of an Orbiting Fire Buckaroos Hat (18-23 keys)

[url]http://www.tf2outpost.com/trade/9059241[/url] 15 keys

[url]http://www.tf2outpost.com/trade/9001479[/url] TS Bills and 4 keys (13 keys)

This is my first suggestion ever, so please let me know if I need to change something.