My recent sale:

Sold for 15 csgo gamma keys on tf2 trade server (csgo>tf2...) // proof http://imgur.com/a/iZLqw

b/o was 16k ( http://www.tf2outpost.com/trade/28454802#latest ) I think 16 keys will be ok.

2#

http://backpack.tf/profiles/76561198018427133#!/compare/1470873600/1472256000 bought on market?

3#

http://backpack.tf/profiles/76561198142082556#!/compare/1470528000/1470700800 bought for 11k+37ref // rounded 13keys

http://www.tf2outpost.com/trade/28387509 b/o was 14 keys.

Seller: http://backpack.tf/profiles/76561198156074887#!/compare/1470009600/1470096000 bought it for ~14 keys mixed.

---------------------------------------------------------

Didn't find more proofs/sales. (proof was founded by original id of unu. in other suggestions)

Set range 13-16 keys.