The Grey Rain Cloak is created through the performance of a minor, undescribed ritual.
Like many others in the game, rituals are used to provide different commodities, from summoning spirits to improving mounts to - in this case - creating a cloak from rainwater.
Since most magic items can be crafted with a ritual, we can easily figure out by comparison the price of the undescribed ritual needed to craft the cloak. The description of the cloak itself tells us which additional conditions are needed.
I'll leave to you the homebrewing of this new ritual, according to your own tastes and balance needs.
There's a second problem, though, and it's that this peculiar cloak is supposed to be important for the character, being a gift from his father.
You could essentially go two ways.
Since the cloak is not listed under magic items, have the Make Whole ritual being able to restore it; or homebrew a new ritual that can refill (at a price proportionate to the missing lenght) the cloak.
Should you choose this last option, carefully consider that your stingy player is not going to be happy if you price this ritual so high that refilling let's say 50% of the cloak costs more than 50% of making a new one; while a ritual that allows for refilling instead of crafting anew should probably cost a little more for the improved utility.
@DampeS8N makes me notice how you could use the crafting ritual flavoring it as a refill. The same cost-advantage considerations apply.