The hat used for the last suggestion is now duped ( http://backpack.tf/item/3904337645 http://www.tf2outpost.com/item/440,3983715331,631,5 ) so no recent valid sale on that one.

A clean Vivid Plasma Hat with no name and a specialized Australium black box got bought recently for a duped Roboactive Hat with no name.

RA hwnn mini:

sold recently for Harvest Grenadier Softcap (74) + 40 keys pure

http://www.tf2outpost.com/trade/27219967 (offer from MadDisabledKid)

the seller had a quicksell b\o of 90 keys and he took the offer as equivalent of 90 pure.

So the mini on the RA HWNN is 90.

aussie black box mini:

quote ApothicЯ aka PetitCœur previous owner of the bb

"got it a month unsold at 19 keys, got multiple times offered 18 pure, so not worth 20 keys

The australium black box is around 18-18.5 keys as I explained above.

So Vivid Hat with no name should be 90 = x + 18.25 -> x = 72 keys

thanks to ApothicЯ aka PetitCœur and Foamy the Fearsome for the help :P