Can you use true polymorph to turn someone into a spell component? nowhere in the spell does it say anything about gold cost, only size.

The transformation lasts for the duration or until the target dies or is destroyed,

Thinking on this, I'm really not sure what you're trying to get at. Are you trying to get a free spell component out of a 9th level spell slot? I'm not sure if that is the best use of that slot. Why not just wish for the spell effect of the spell you wanted to cast anyway?

Are you just trying to make a creature disappear by turning them into a spell component then using that spell component for a spell? If so, see my previous answer.

I see no reason why you can't transform a creature into a spell component, when turning a creature into an object they take on the stat block of that item, which presumably means the GP cost too. However it does beg the question of what occurs if you transform somebody into a component that is expended/destroyed as part of the spell's casting, if a DM even allows that.

Sorry I should have been more specific, I ment indeed to turn them into a spell component that get's consumed by the spell. And yes it is a very elaborate way of achieving a goal that could get achieved way easier

Ah. see post #2 then. A spell consumes the component, thereby destroying it.