All the hats I've been pricing I've owned. It helps to have a few sales before tracking down more.

Sale 1:

http://backpack.tf/profiles/76561198052516999?time=1445670000&compare=1446188400

None of the histories match. The Dread Knot is outdated and I can't find a mini. I don't even know if a sale occurred here. Not using.

Sale 2:

http://backpack.tf/profiles/76561198100409323?time=1446015600&compare=1447488000

This one is confusing. Looks like it sold for a Smoking Trophy Belt, a MW Strange Spec KS Night Owl, a key and a few hats. The Smoking Trophy Belt is outdated too and I can't find a mini either. The only two people on Outpost are asking for ridiculous prices (20 and 30). Not using.

Sale 3:

http://backpack.tf/profiles/76561198096210931?time=1446796800&compare=1446710400

Sold for 10 keys, ~13.5 USD worth of kits, and 30.77 metal.

x = 10 + 4.89 + 1.9 (keys were 16.11 back then)

x = 16.8, rounding up to 17. Using as low end.

Sale 4:

http://backpack.tf/profiles/76561198096210931?time=1446883200&compare=1446969600

Sold for V. Plasma Topi.

Topi mini:

Sold for a Miami Nights Conquistador and 15 ref (The Conquistador was offered)

Conquistador mini:

http://imgur.com/lNwq25S , http://imgur.com/HTWD0AV

Offered on a TaW Degroot Experience (21.5)

Keys back then were 16.11 ref, so the math is

21.5*.9 + 0.93 = x

x = 20.28, rounding down to 20.

Back to the Dragonborn Helm:

x = 20 * 0.9

x = 18

Can't find any other sales. Going with a range of 17-18. Feel free to tell me if I did anything wrong in the comments!

seller at 19: http://www.tf2outpost.com/trade/27547843

seller at 18: http://www.tf2outpost.com/trade/27562801

supports a drop