>2 years old price!

Sale 1:

Sold for Bubbling Blighted Beak (18.5*0.9) = 16.65 (rounding to 17 keys)

http://www.tf2outpost.com/trade/25756024

http://backpack.tf/profiles/76561198045370812?time=1430290800 - (29th April)

http://backpack.tf/profiles/76561198045370812?time=1430377200 - (30th April)

~The 2 Robo Keys + 9 ref were used to buy the Silver Botkiller Flamethrower with parts attached.

Sale 2: (outlier & unpriced hat)

Sold with Bubbling Hustler's Hallmark for Poisoned Shadows Outdoorsman

http://www.tf2outpost.com/trade/25951801 | http://www.tf2outpost.com/trade/25958922

http://backpack.tf/profiles/76561198135245669?time=1431586800 - (14th May)

http://backpack.tf/profiles/76561198135245669?time=1431673200 - (15th May)

Notes:

No other recent sales. Sale 1 cannot be considered an outlier due to the fact that it took the seller more than 9 months to sell it: http://www.tf2outpost.com/trade/21591660

Furthermore, the seller has literally not got a single offer on his hat for numerous months, as well as his B/O being 25 keys: http://www.tf2outpost.com/trade/22594647

Conclusion:

One valid sale at 17 keys. Taking a final price of 17 keys.

Feel free to support/counter my suggestion!