Taking buds as 21 keys. Updating with a better range thanks to polar's comment: http://backpack.tf/vote/id/523f02ec4dd7b86d618b456b

http://www.tf2outpost.com/trade/14321792 B/O: 7 buds. Sold yesterday for http://i.imgur.com/FHyxbNm.png --> Burning Bombing Run (~3.1 with overpay, original suggestion 3.3 - 3.5), Beams Blokes (~2 with overpay based on original suggestion), 1 bud, 14 keys and gold mk 1 scatter with parts (5-6 keys) --> 7 buds.

http://www.tf2outpost.com/trade/13422767 B/O: 7 buds pure or 7.5 in unusuals. Sold for Secret Lid (7.1 buds with overpay).

http://www.tf2outpost.com/trade/12934325 B/O: 7 buds. Sold for 7 buds.

http://www.tf2outpost.com/trade/13315210 B/O: 7 buds. Sold for Searing Plasma Bonk Boy (4.68 buds with overpay), 1 bud and 22 keys pure ~ 6.68 buds.

Also, one available on the market for 7 buds flat:

http://www.tf2outpost.com/trade/14369462