Way, WAY out of date, but last suggestion did mention 40 keys pure, and this one actually has evidence of it (much later, but still!).

Hats that have moved:

Hat 1: http://backpack.tf/item/1818834827

Hat 2: http://backpack.tf/item/2551654371

All sales listed in order from youngest to oldest for each hat.

Sales (Hat 1):

40 pure:

http://backpack.tf/profiles/76561198127971615#!/compare/1462492800/1462579200

= 40

Prior two sales on this hat don't properly show up in the snapshots.

Maaaaaybe 28 keys, but very unclear. Quite a bit of other stuff moved.

Seller:

http://backpack.tf/profiles/76561198230758044?time=1461456000

http://backpack.tf/profiles/76561198230758044#!/compare/1461456000/1462406400

Buyer (totally useless):

http://backpack.tf/profiles/76561198208062409#!/compare/1461456000/1461542400

One before it, who knows:

Seller (10-day gap):

http://backpack.tf/profiles/76561198130462794#!/compare/1460937600/1461801600

Buyer:

Whole bunch of stuff?

http://backpack.tf/profiles/76561198230758044#!/compare/1460937600/1461024000

Sales (Hat 2):

Sale:

Bought off steam market:

Buyer:

http://backpack.tf/profiles/76561198123100066#!/compare/1461888000/1462060800

Seller:

http://backpack.tf/profiles/76561198033058187#!/compare/1461715200/1461801600

Was sold approximately 4/29; unfortunately, 4 unusual cold killers sold during that period. No way to verify any of them without buyer/seller, though the average was $78.

Buyout was 40 keys: http://www.tf2outpost.com/trade/28090340

Sale:

Don't 100% trust this one, since it went back and forth a couple times (could be an alt?), but could be just traders that do a lot of business, and the numbers do seem about right.

TF Logo Cold Killer

+ KaW Master's Yellow Belt (30)

+ Cool WW Night Owl (60-62, http://backpack.tf/suggestion/57623e8183de6b0ece1c9223 )

+ Spec KS Aussie SMG (8 keys, selling/buying same as non-KS)

+ 2 keys

for

Smoking Virtual Viewfinder (136)

http://backpack.tf/profiles/76561198246546920#!/compare/1464048000/1464134400

= 35

Sale is considerably less trustworthy than the pure sale, using it as general support but not including in price.

Sale:

Sold with KaW Master's Yellow Belt (30) for Frostbite Front Runner (63)

http://backpack.tf/profiles/76561198246546920#!/compare/1464134400/1464480000

= 33

Sale:

Sold, again with KaW Master's Yellow Belt (30) and 2 keys...

for Duped Cauldron Bubbles Samur-Eye (90, wildly out of date)

http://backpack.tf/profiles/76561198125855246#!/compare/1464393600/1464566400

= ...It is a mystery

Summary: Sales at 33, 35ish, and 40, pure sale anchoring high end.

Still, would not be surprised if this can move for the high end more often, the two lower sales are a single trader's buy and sell.