Crawled back through a month's worth of trades for this price. No previous suggestions.

http://www.tf2outpost.com/trade/9614285

Ghosts Milkman = 3.5 - 4.5 buds

Stormy Company Man + 15 keys = 3.8 - 4.3 buds

NOTE: The Moon Pith Helm is not a 1:1 offer with the Chieftan. The same guy offered his Pith Helm for another Chieftan + 2 buds (see last link).

http://www.tf2outpost.com/trade/9790554

Stormy Storm Brainiac + 4 keys = 4 - 4.3 buds

Ghosts Crown + 1 bud = 3.8 - 4.6 buds

http://www.tf2outpost.com/trade/9563001

SOLD FOR Secret Jeepcap - 1.2 buds = 4.4 - 5.9 buds

http://www.tf2outpost.com/trade/9561447

Would have accepted 4 bud offer.

The last sale I found was at around 4.6 - 4.8 buds considering unusual overpay (the Secret Jeepcap). There are consistent offers around the 4 - 4.2 bud range, but nothing really above that. Thus it seems like the last sale was a major ripoff for the buyer since nobody was even close to offering that much.

Lower bound at 3.8 buds to account for unusual overpay, and higher end at 4.3 because sometimes people offer Secret Jeepcaps.