Proof below (3 sales).

Never had a price before, no suggestions accepted so far.

http://www.tf2outpost.com/trade/13685075

-sold for 3 buds

http://www.tf2outpost.com/trade/13193711

-do cosa, b/o 9 buds

-dp cosa (7.5) sold for cauldron pyromancer+cauldron ten gallon+p. fetti pyromancer=3 buds taking mid prices as it was offered

http://www.tf2outpost.com/trade/13246616

-wanted almost 4 buds, asked buyer to add

-sold for 3 buds+3 keys=~3.1+ buds, according to bp history but unclear as he sold both his dp cosa and pyromancer:

http://backpack.tf/id/76561198036351515?time=1375167600 before

http://backpack.tf/id/76561198036351515?time=1375254000 after

Sales: 3, 3, 3.1

All sales fit into suggested range. Low end based on the first 2 sales and the high end from the last sale.