The low end of this hat's price was only 2.2 buds before the currency conversion.

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Sold for Aces High Front Runner (26) + 3 keys:

http://backpack.tf/profiles/76561197991184877?time=1430982000 (May 7th)

http://backpack.tf/profiles/76561197991184877?time=1431068400 (May 8th)

26 * .9 + 3 = 26.4 keys

Rounded to 26 keys

Sold for 28 keys (?):

http://www.tf2outpost.com/trade/25670524

http://backpack.tf/profiles/76561198025613718?time=1429858800 (April 24th)

http://backpack.tf/profiles/76561198025613718?time=1429945200 (April 25th)

I could not track the exact keys, but seller had a buyout of 28 keys for 2 weeks regardless.

Sold for 5 buds (5.5):

http://backpack.tf/profiles/76561198021272428?time=1428476400 (April 8th)

http://backpack.tf/profiles/76561198021272428?time=1428649200 (April 10th)

http://backpack.tf/profiles/76561198025613718?time=1428649200 (April 10th)

http://backpack.tf/profiles/76561198025613718?time=1428735600 (April 11th)

5 * 5.5 = 27.5 keys

Rounded to 28 keys

Sold with Skill Gotten Gains Taunt: Spent Well Spirits (42) for Strange Professional Killstreak Australium Eyelander (45):

http://backpack.tf/profiles/76561198159509740?time=1430636400 (May 3rd)

http://backpack.tf/profiles/76561198159509740?time=1430895600 (May 6th)

SGG Spent Well Spirits is outdated and is probably ~20 keys, which would result ~25 keys for the Miami Cowl. Estimation here should be close.

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Strange Professional Killstreak Australium Eyelander:

http://backpack.tf/vote/id/55522d6fba8d8893618b4567