Currently Mark of potency rank 7 is cheaper than rank 6, due the lack of demand. One is used used in rank 15 regular enchants.
Weapon and armor enchant need 10 Marks of potency rank 6 to upgrade from 12 to 13.
And
15 marks rank 6 to upgrade from 13 to 14.
I suggest to make the 13 to 14 upgrade of armor and weapon enchants take 12 marks of rank 7 instead of 15 rank 6.
This is the exact total sum, but it will create some demand to rank 7 marks, so the market doesn't look silly.
@asterdahl In short:
Current 13->14
15 MOPS 6
Suggested 13->14
12 MOPS 7
Comments
The OP is correct tho, in that MP 7s are cheap as chips because they are not nearly as in demand as the lower ones.
I have simply stopped upgrading weapon and armour enchants now besides taking so many refinements the 1% forces you to use a Coalescent even when upgrading lower ranks. It's simply not worth the cost.