Easy to find it for less than 5 keys. Even selling for 4 + sweets is difficult. People aren't willing to pay more than 4 keys. Some was sold at 3 + sweets. Proof is below. 3 keys seems like a quicksell price.

Sellers:

http://www.tf2outpost.com/trade/8398031 - 5 keys, unsold, 1 week

http://www.tf2outpost.com/trade/8482081 - 5 keys, unsold, 3 days

http://www.tf2outpost.com/trade/8480409 - 5 keys, unsold, 3 days

http://www.tf2outpost.com/trade/8513855 - 5 keys, unsold, 2 days

http://www.tf2outpost.com/trade/8500121 - 5 keys, unsold, 2 days

http://www.tf2outpost.com/trade/8545443 - 5 keys, unsold, 1 day

http://tf2tp.com/trade.php?trade=9124185 - 4 keys + 2 ref, unsold, 2 days

http://www.tf2outpost.com/trade/8478055 - 4 keys + 1.33 ref, unsold, 3 days

http://www.tf2outpost.com/trade/8543717 - 4 keys + 1 ref, unsold, 1 day

http://www.tf2outpost.com/trade/8540640 - 4 keys + 1 ref, unsold, 1 day

http://www.tf2outpost.com/trade/8565817 - 4 keys + 1 ref, still new

http://www.tf2outpost.com/trade/8455546 - 4 keys, unsold, 5 days

http://www.tf2outpost.com/trade/8455546 - 4 keys, unsold, 1 day

http://www.tf2outpost.com/trade/8530865 - 4 keys (with robots destroyed attached), unsold, 1 day

http://tf2tp.com/trade.php?trade=9129991 - 4 keys, 10 hours, no takers

http://tf2tp.com/trade.php?trade=9130603 - 4 keys, sold

http://www.tf2outpost.com/trade/8398677 - sold for 3 keys + sweets

http://www.tf2outpost.com/trade/8462563 - accepted an offer 3 keys + 1.66 ref

Buyers:

http://www.tf2outpost.com/trade/8245936 - 4 keys, bought

http://www.tf2outpost.com/trade/7795240 - 4 keys, bought (comments)

http://www.tf2outpost.com/trade/7974248 - declined 4 keys offer

http://www.tf2outpost.com/trade/8299474 - 3 keys, bought at least 1 ( http://www.tf2outpost.com/item/440/1444019867 )