Dark Matter should be returned to gathering points.

[Vaer]

Looking at it a bit closer,

ilvl70 weapon (Morbid Mogblade) Right now it's at 68% and costs 160 gil to repair from vendor.

160gil / 32 = 5gil
5 x 100 = 500gil

Assuming every 1% is a static fee, it's roughly 5 gil per 1% or 500 gil to repair it completely from broken assuming that each 1% is 5 gil all the way down to 0 and it doesn't increase and there isn't a penalty if it's completely broken.

With DM5 at 350gil at NPC, it's still cheaper (around 30% cheaper to repair with DM) and will probably get cheaper if you can get it for a discount from players. Looked at an iLvl52 shield and it's roughly the same 5 gil per 1%.