This was gonna be a mini but I found so many sales that I figured I'd have a go at suggesting it. Resuggesting with better range.

Sale 1

Sold for green black hole defiant spartan 49-55 keys = 52 * 0.9 = 47 keys

Buyers bp before: http://backpack.tf/profiles/76561198107367025?time=1427353200

Buyers bp after: http://backpack.tf/profiles/76561198107367025?time=1427439600

Sale 2

Sold/offered with 6 buds (~10 keys each at time of sale) for cloud 9 viewfinder (165-173 keys) = 169 - 60 = 109 keys

Sellers bp before: http://backpack.tf/profiles/76561198042664154?time=1426834800

Sellers bp after: http://backpack.tf/profiles/76561198042664154?time=1426921200

Sale 3

Sold for killawatt el jefe (55-64 keys) and miami company man (28-35 keys) = (59.5 + 31) * 0.9 = 81 keys

Sellers bp before: http://backpack.tf/profiles/76561198084574728?time=1426316400

Sellers bp after: http://backpack.tf/profiles/76561198084574728?time=1426575600

Sale 4

Sold for memory leak human cannonball (56 keys), memory leak tam o shanter (21-23 keys) and haunted ghosts hard counter (13-14 keys) = (56 + 22 + 13.5) * 0.9 = 92 keys

Sellers bp before: http://backpack.tf/profiles/76561198083834822?time=1425970800

Sellers bp after: http://backpack.tf/profiles/76561198083834822?time=1426230000

Sale 5

Sold for overclocked kiss king 69 * 0.9 = 62 keys

Sellers bp before: http://backpack.tf/profiles/76561198087925171?time=1425283200

Sellers bp after: http://backpack.tf/profiles/76561198087925171?time=1425628800

Sale 6

Sold for blizzardy capo's capper (12 keys), 2 max's heads (37 keys each at time of sale), bill's hat (3 keys at time of sale) = 80 * 0.9 = 81 keys

(this trade is a bit confusing though, lots of items moving back and forth between backpacks so let me know if I missed anything)

Buyers bp before: http://backpack.tf/profiles/76561198039661263?time=1428044400

Buyer's bp after: http://backpack.tf/profiles/76561198039661263?time=1428390000

Sale 7

Appears to have sold for 25 keys pure but I may be missing something because wtf. Outlier either way

Sellers bp before: http://backpack.tf/profiles/76561198071921939?time=1427823196

Seller bp after: http://backpack.tf/profiles/76561198071921939?time=1427871600

Sale 8

Appears to have sold for orbiting fire crone dome 78 * 0.9 = 70 keys

Sellers bp before: http://backpack.tf/profiles/76561198042258870?time=1425884400

Sellers bp after: http://backpack.tf/profiles/76561198042258870?time=1425970800

Sale 9

Sulphrous titanium tower (56-69 keys), 3 keys and a pro sniper kit (~15 keys) = (62.5 + 15)*0.9 + 3 = 73 keys

http://www.tf2outpost.com/trade/25259902

Buyers bp before: http://backpack.tf/profiles/76561198042258870?time=1425628800

Buyers bp after: http://backpack.tf/profiles/76561198042258870?time=1425801600

Mini for orbiting fire crone's dome:

Sold for max's (40 keys at time of sale), 2 buds (12 keys at time of sale), and 14 keys = ~ 78 keys

Buyers bp before: http://backpack.tf/profiles/76561198068174207?time=1424937600

Buyers bp after: http://backpack.tf/profiles/76561198068174207?time=1425024000

This is all over the place, with sales at:

47

109

81

92

62

81

25

70

73

78

47 and 109 definitely seem like outliers, 62 and 92 are a bit closer but still stretch the range out a lot for only 2 sales. Bulk of sales are within 70-81.

Feedback welcome!