http://www.tf2outpost.com/trade/28808498 B/O 65 keys. His buyout was even lower but I can find it.

http://backpack.tf/item/3315793181 unsold for over 1 month

I think it has to be at least 15% lower then 75 keys, So im not using it.

sale 1)

http://backpack.tf/profiles/76561198126679929#!/compare/1495324800/1495411200

sold for a DUPED scorching flames big country 70k+Aussie wrench 19k= 89

DUPED Big country mini suggestion

http://backpack.tf/profiles/76561198067575136#!/compare/1495238400/1495324800

sold for a armarathine generals 60 keys+10 key aussie=70 keys

Credits to Marty Birdman

http://www.tf2outpost.com/trade/28973353 - B/O was 65

65+19=84

sale 2) Credits to Marty Birdman

http://backpack.tf/profiles/76561197990986202#!/compare/1494720000/1494806400

sold for 48 keys pure. 48 keys pure was his b/o, but I can't find the link.

sale 3) Credits to Marty Birdman

http://backpack.tf/profiles/76561198182552231#!/compare/1496016000/1496534400 - Was sold on the steam market

http://steamcommunity.com/market/listings/440/Unusual%20Heer%27s%20Helmet - Sale was for $207.09 / ~2.46 (key price) 207.09÷2.46~84 keys

Credits to K!ng

Currently listed for $140 (~69 keys, taking keys at $2.04) on marketplace, and has been there for a while.

https://marketplace.tf/items/30378;5;u90

https://backpack.tf/item/3277543281

Sale 84, 69, 48 and 84.

48 is too low

Final price=rounding to 70-85