3 month old price, 2 for sale on OP, lots in the wild.

http://backpack.tf/unusuals/480/Tam_O%27_Shanter for reference

Trades:

http://www.tf2outpost.com/trade/9268159

-18 keys + TS bills (~27 keys)

http://www.tf2outpost.com/trade/7007921

-1 bud (3x)

-30 keys

-26 keys

Sales:

http://www.tf2outpost.com/trade/7990168 - 1 bud, maybe sweets? (1 month ago)

So it appears that 27-30 keys gets offered a lot. I'm setting the range at 30 keys +/- 10% since 30 is the highest pure offer and it was refused. Since there are numerous 1 bud refused offers and a sale at 1 bud, this range appears to be fair to both buyers and sellers.

Please add your additional proof if you know of any. Thanks!