Eight inner gates give you up to 202 initiative per quick rune added. That alone make the tens of millions coins spent there worth it. Gold and red runes for the 5 basic stats give a very big boost if brought to level 9 and secondary stats like damage reduction and combo you can add there are all but useless. But if your comparison is with spending coins on equipment strengthening then yes, as rough power gained the latter is better, but since initiative means to have or to not have the barrier up then you understand why inner gates is way better.