Verified Solds:

http://www.tf2outpost.com/trade/17379664 for Green Energy Bonk Helm : 3 buds - 10 % is 2.7 buds

Notable Rejections:

http://www.tf2outpost.com/trade/15110251 Phosphorous Dr. Dapper's Topper : 2 buds

http://www.tf2outpost.com/trade/18238965 Haunted Ghosts Dead Cone: 38 keys

http://www.tf2outpost.com/trade/17297273 rejected purple energy billy cock: 2.8 buds

Since there are currently only 3 hats in existence, but two are currently for sale on outpost with a B/O of 2.5 buds and 5 buds and one is for sale on bp.tf classified for 3.5 buds. Although the only verified purchase I could find is around 2.7 buds pure, the fact someone is willing to sell it for 2.5 buds pure right now should be the standard. There are also multiple examples of offers of below two buds being flatly rejected, so for a first price, 2.5 buds is rather fair.

As always please feel free to add any suggestions to help pin the price down for this. This suggestion isn't perfect, but it's much better than nothing.