Its price hasn't been updated for 5 months, here I go my suggestion:

There are only 2 pages selling threads and 1 page buying threads. But no relevant buying threads(lowballers, quick buying ask for less then 3 keys and they are all unsuccessful)

Selling for 3 keys:

http://www.tf2outpost.com/trade/10507023

Sold for 3 keys:

http://www.tf2outpost.com/trade/10141668 : item no longer exists in seller's bp, http://www.tf2outpost.com/backpack/176239 , but its in buyer's bp, http://www.tf2outpost.com/backpack/318354

http://www.tf2outpost.com/trade/10529218 : sold for festive scatter (3 keys)

Unsuccessful sales higher then 3 keys:

-3 keys +: http://www.tf2outpost.com/trade/10528484

-3 keys 1 ref: http://www.tf2outpost.com/trade/10503714+

-3 keys 2 ref: http://www.tf2outpost.com/trade/10440549 - notes: asked for 3.5keys initially, buyer refused his counter-offer(3 keys 2 ref)

-3.5 keys: http://www.tf2outpost.com/trade/10531435 ,

-3 keys 4 ref: http://www.tf2outpost.com/trade/10529218

Conclusion: Although there is an unsuccessful trade selling for 3 keys, I would still suggest 3 keys, since there are 2 successful trades for 3 keys, and no successful trade higher then 3 keys.