I think the cost of customizing/fitting robes/clothing would be significantly less than for armor, for obvious reasons. The rules currently reflect that clothing is easier to make fine and exceptional on pg 238:
Quote:
Clothing and Accessories
The prices for clothing are generally for average apparel, with a few exceptions. You may purchase Fine or Exceptional quality versions of clothing for 5x or 10x the normal value. The listed encumbrance values are when worn; all clothing has an Encumbrance of 1 while being carried.
And I would think the same logic would apply to the cost of customization, as it's easier to modify cloth than steel. So instead of it costing one third of the cost of the clothing to customize, I would think it would cost somewhere between a half and a tenth of that. So I'd guess in the 5-10% of cost of the clothing, but that's just my thoughts.
I'm no one important. You'll need an official call.