Can't find any sold for 6 keys. Fairly easy to get for 5.5 (just check classifieds) and even possible to get for 5 keys, although it is fairly difficult.

6 keys

http://www.tf2outpost.com/trade/8331626 - Unsold for 6 keys

http://www.tf2outpost.com/trade/8314949 - Unsold for 6 keys

http://www.tf2outpost.com/trade/8307646 - Unsold for 6 keys

http://www.tf2outpost.com/trade/8118994 - Unsold for 6 keys

http://www.tf2outpost.com/trade/8213926 - Unsold for 6 keys

http://www.tf2outpost.com/trade/8232778 - Unsold for 6 keys

http://www.tf2outpost.com/trade/8308438 - Unsold for 6 keys

http://www.tf2outpost.com/trade/8280944 - Unsold for 6 keys

http://www.tf2outpost.com/trade/8315944 - Unsold for 6 keys

5.5 keys

http://www.tf2outpost.com/trade/8382129 - Selling for 5.5 keys

http://imgur.com/oCrNi7o - Selling for 5.5 keys

http://www.tf2outpost.com/trade/8205925 - Offer of 5.5 keys

5 keys

http://www.tf2outpost.com/trade/8382003 - C/O of 5 keys

http://www.tf2outpost.com/trade/7922851 - Selling for 5 keys (1 in stock)

http://imgur.com/kodWaAx - Selling for 5 keys

Seems like it's readily available at 5.5, and 5 is possible with enough effort. If anyone can come up with sufficient counter proof I'll close this and open 5 - 6. It's always hard when sold trades aren't displayed.