Looks like this thing is dropping

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Sellers @ 4-4.33 Refined & Below

Backpack.tf Sellers

http://backpack.tf/classifieds/search/breather%20bag

4 @ 4.33

1 @ 4 Refined

2 @ 3.66 Refined

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Selling @ 4-4.33 & Below

Outpost Sellers

http://www.tf2outpost.com/trade/12235594 (4.33 Refined)

http://www.tf2outpost.com/trade/12222893 (4.33 Refined)

http://www.tf2outpost.com/trade/12220010 (4.33 Refined)

http://www.tf2outpost.com/trade/12216353 (4.33 Refined)

http://www.tf2outpost.com/trade/12225301 (4.33 Refined, Sold!)

http://www.tf2outpost.com/trade/11990199 (4.33 Refined)

http://www.tf2outpost.com/trade/12174308 (4.22 Refined)

http://www.tf2outpost.com/trade/12205093 (4.22 Refined)

http://www.tf2outpost.com/trade/12230110 (4 Refined)

http://www.tf2outpost.com/trade/12223025 (4 Refined)

http://www.tf2outpost.com/trade/12221406 (4 Refined)

http://www.tf2outpost.com/trade/12203599 (4 Refined)

http://www.tf2outpost.com/trade/12101668 (4 Refined)

http://www.tf2outpost.com/trade/12221634 (4 Refined)

http://www.tf2outpost.com/trade/12199696 (4 Refined, Sold!)

http://www.tf2outpost.com/trade/12223227 (3.66 Refined)

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Unsold @ 5 Refined

http://www.tf2outpost.com/trade/12203895

http://www.tf2outpost.com/trade/12187317 (Breather + 2 Rec for 1 Key)

http://www.tf2outpost.com/trade/12051771

http://www.tf2outpost.com/trade/12223740

http://www.tf2outpost.com/trade/12192783

http://www.tf2outpost.com/trade/12207403

http://www.tf2outpost.com/trade/12206159

http://www.tf2outpost.com/trade/12097546

http://www.tf2outpost.com/trade/12106854

http://www.tf2outpost.com/trade/12192501

http://www.tf2outpost.com/trade/12140794 (5 Refined, painted orange)

http://www.tf2outpost.com/trade/12193076 (4.77 Refined)

Unsold @ 4.66 Refined

http://www.tf2outpost.com/trade/12027520

http://www.tf2outpost.com/trade/12221025 (still new)

http://www.tf2outpost.com/trade/12155517

http://www.tf2outpost.com/trade/12210957

http://www.tf2outpost.com/trade/12217445 (Still new)

http://www.tf2outpost.com/trade/12203786

http://www.tf2outpost.com/trade/12144962

http://www.tf2outpost.com/trade/12166428

http://www.tf2outpost.com/trade/12211203 (4.67 Refined)?

http://www.tf2outpost.com/trade/12176188

http://www.tf2outpost.com/trade/12160129

Looks like @ 5 Refined is mostly impossible to get sold, @ 4.66 is not likely to get sold. Sellers @ 4.33 Refined still have a chance of getting sold, and there are people selling it less than 4.33 Refined, so 4-4.33 seems about a good range.