why in the world is an ebony helm worth 15k when the armor rating is less than my dark brotherhood mask that is worth 200?

First of all, the value of items aren't derived solely on their effectiveness.

Secondly, check your skill levels, you probably have higher Light Armor than you do Heavy Armor, it directly affects the defense ratings.

DBH armour = light

Ebony = heavy

Your light armour skill as Richie said is probably higher. Daedric has the highest AR with heavy armour maxed at 100. Then Ebony. Glass armour with level 100 light armour is the best light armour but ranks 3rd overall in best AR

As to the materials. Think about it. Ebony armour is derived and made from Raw Ebony Ore. DBH armour is merely leather..already you have the difference of metallic VS animal hides