Returning to my first accepted suggestion.

http://backpack.tf/item/3260584798

Sale 1) Sold for a KS Aussie Amby (~18) + 5 Keys in Items (Bill's was more at the Time)

http://backpack.tf/profiles/76561198043592289?time=1431068400&compare=1431414000

23 * 0.9 = ~21 Keys

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Sale 2) Sold + 4.5 Keys of Items for an Aussie Grenade (19 then)

http://backpack.tf/profiles/76561198074910378?time=1431327600&compare=1431586800

X + (4.5 * 0.9 = ~4) = 19 Keys

X = 15

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Sale 3) Sold for a Pro KS Gold Botkiller Rocket Mk.I (~15) * 0.9 = 14 Keys

http://backpack.tf/profiles/76561198120143871?time=1437721200&compare=1437894000

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Ruling 21 an outlier since it's the oldest sale.

The sales have been on a downward trend since then.