I don’t think I understand the question. If something is expensive but will definitely (let’s say with 99% certainty) save my life (which I think is the sort of thing you are describing as expensive_but_must), I would buy it at almost any cost.
I don’t think I understand the question. If something is expensive but will definitely (let’s say with 99% certainty) save my life (which I think is the sort of thing you are describing as expensive_but_must), I would buy it at almost any cost.