I'll crack my hand at this. (meaning, I'll try to elaborate on the previous suggestion)

Sale 1) http://www.tf2outpost.com/trade/25549678 --- Bought for GBH Plumber (19 * 0.9 = ~17) + Pro KS Silver Minigun Mk. II (8) = 25 Keys

http://backpack.tf/profiles/76561198027298961?time=1427835458

Recent Sales on the Minigun (On SCM) = $23.78 * 0.85 = 20.21 / 2.4 = ~8 Keys

Plumber Mini

http://backpack.tf/profiles/76561198039666177?time=1429772400 -- Sold for Planets Virus (~19)

Sale 2) http://www.tf2outpost.com/trade/25549678 --- Quicksold for 27 Pure (says last Suggestion)

Sale 3) http://www.tf2outpost.com/trade/25581775 --- Sold for 24 Keys (says last Suggestion)

Sale 4) http://www.tf2outpost.com/trade/25729626 --- Sold for Bubbling Scopers (Outdated @23 ||| Mini @ 20 * 0.9 = 18) - Outlier

http://www.tf2outpost.com/item/440,3708836843,980,5 - Scopers

Scopers Mini

http://www.tf2outpost.com/trade/25883174 --- Seller @ 20 (Unsold for 6 Days)

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Summary

25

27 - Quicksell

24

18 - Outlier

Rounding 24 to 25 and 27 to 25.

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Mistakes? Changes I should make? Comment below.

Thanks! ~Spy