Just tweaking this a bit to make the range a bit more accurate

Sale 1

Sold for chiroptera venenata your worst nightmare (66-88) = 77 * 0.9 = 69 keys

Seller before: http://backpack.tf/profiles/76561198081076110?time=1430809200 (May 5th)

Seller after: http://backpack.tf/profiles/76561198081076110?time=1430895600 (May 6th)

Sale 2

Sold for for csgo knife and gun = ((106 + 16)* 0.9) to ((125 + 19)* 0.9) = 110 - 130 keys

http://www.tf2outpost.com/trade/25803627

Knife

http://steamcommunity.com/market/listings/730/%E2%98%85%20Karambit%20|%20Night%20%28Minimal%20Wear%29

Average buyer pays ~$300

Average seller receives ~$255

Key price ~$2.40

300/2.40 = 125 keys

255/2.40 = 106 keys

Range = 106-125 keys

Gun

http://steamcommunity.com/market/listings/730/AK-47%20|%20Vulcan%20%28Minimal%20Wear%29

Average buyer pays ~$45

Average seller receives ~$38.25

Key price ~$2.40

45/2.40 = 19 keys

38.25/2.40 = 16 keys

Range = 16-19 keys

Sale 3

Part of a bulk trade for mallard sleeveless in siberia - not using

Buyer before: http://backpack.tf/profiles/76561197997855531?time=1427612400 (March 29th)

Buyer after: http://backpack.tf/profiles/76561197997855531?time=1427698800 (March 30th)

Mini for cv your worst nightmare:

http://backpack.tf/vote/id/554cef35b98d88353c8b45cb

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Offers

http://www.tf2outpost.com/trade/25890833

Haunted phantasm jr aviator assassin + ~20 keys in sweets = ~110

DaD bear necessities + DaD tartan spartan = ~150

http://www.tf2outpost.com/trade/25863872

Stare from beyond aviator assassin - unpriced but likely >100

Clean CV spartan+clean steaming bomber = >100

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Offers above 100 suggest that the higher sale is more accurate, so using that. Feedback welcome.