For every crafting aptitude you gain, the amount of mats it takes to make items should decrease.
If you have made 10,000 ironwood bows… why would it take more than 1 ironwood plank to get it right the first time?
Of course we would have to adjust aptitude exp requirements to balance it out.
This system would reward crafters for crafting items by lowering their cost to craft.
And for gathering, aptitude could increase yield and or luck for rare items.