All recent sell listings both fulfilled and outstanding have been 10-15 keys. This price falls in line with recent pricing on the Steam Marketplace for the Wispy Halos unusual taunt after the 15% fee. This is further supported by the lack of trades to bots who are aiming to buy the taunt for ~6 keys or fewer. The current price on backpack.tf is $30 which is roughly equivalent to 17 keys with room for inflation and deflation.