The way the game handles ingredients, beyond the necessary ritual link, is through declarations. So if you wanted to buy an expensive component for the spell, you'd make a declaration with resources. If you failed that declaration, one possibility is that you realize what you need is more expensive than what you can afford, so you have to find alternative ingredients. Or you use the skip a scene rules to replicate the time it takes your character to shop around for a better price or to pull money out of accounts or whatever.
Basically, you don't know how expensive a spell is going to be until after the caster decides to bring resources into it. Until then, you just assume that the monetary expense is not noteworthy.
