Proof:

http://www.tf2outpost.com/trade/14934757 - Sold for 11 keys in less than 16 hours

http://www.tf2outpost.com/trade/14950456 - Sold for 11 key in less than 6 hours

http://tf2tp.com/trade.php?trade=9822084 - Sold SF Grenade with Critical (2), Posthumous (0.33 - 0.44 ref) and Defender (0.33 - 0.44) for 14 keys, since parts lost a lot of value after being attached it sold for 12 - 13 keys

http://www.tf2outpost.com/trade/14928703 - Sold for 12 keys in less than a day

http://www.tf2outpost.com/trade/14915418 - Wanted 12, ended up selling for 7 keys + SF Amby (6) = 13, sold it in less than a day

http://www.tf2outpost.com/trade/14912166 - Sold for 13 keys?

http://tf2tp.com/trade.php?trade=9820799 - Sold for 13 keys

http://www.tf2outpost.com/trade/7879566 - Has offers of 12 keys and Uncraft Flip Flops (10) + 2 keys

http://www.tf2outpost.com/trade/13005724 - Declined TS Graybans (~2) + Dashin Hashshahin (1 - 1.2 at the time) + Brainiac Hairpiece (2.33 ref) + Das Feelingbeterbager (2 ref) + 8 keys + Strange Splendid Screen (1.2 - 1.5 at the time) = ~13 before overpay, even if item overpay still above 11 and declined Strange Grenade with Gib Kills (2 ref) and Dominations (1.5 - 2) + 7 keys, Grenade is over 4 and 7 keys so that's above 11 as well, and he was also offered 11 keys pure and declined

The amount of trades in sets/parts attached is insane, over half is invalid because of that and seems 11 keys go fast while over 11 is possible as well

At the moment of posting Classifieds had 0 below 11 (2 at 10 were gifted and ones at 13 or more has parts, not counting that)

Big range but there are sales/offers at 12 and above and not much of the market is without parts or not in sets