Proofs:

Sales

Sold last week for clean bill's + 9 keys (15 keys)

http://www.tf2outpost.com/trade/14842131

Offers & B/Os

Open trade. B/o 14 keys,

Meteor offered 15 keys a week ago.

http://www.tf2outpost.com/trade/10540412

Open trade. B/O 2 bills (white/black) (14 Keys)

http://www.tf2outpost.com/trade/13706436

Open trade. B/O 18 keys

C/O 14 Keys

Declined Offers: Planets big chief (14-18 keys)

http://www.tf2outpost.com/trade/14893267

The ugly

B/O 21 keys

Last offer was the bill's + 8 keys seen above and other offers are months old.

http://www.tf2outpost.com/trade/9486369

Conclusion

Low is 14 because of B/Os at 14 keys and considered offers at 14 keys.

High 15 because of sales and unusual offers.

Nothing below 14 keys seems to be considered and no one is generally willing to offer above 15 keys.