Magic and POW stats

[Papa Thulhu]

I have a bit of a question, to better understand some spells in CoC. I've been looking over the magic sections to get a better understanding of the Magic system. When things cost POW, is that considered a permanent reduction of POW, no way to retrieve back? Understandable that spells in this world are not to be something to willy-nilly like in DnD, there are serious costs. But say something like the Elder Sign, I recall costs 25 points, pretty steep. So would that POW lost be permanent, and if so, what would be a good way to explain in a way why it costs that? I can understand how to explain these things costing sanity, but unsure how to justify a loss of POW, at a deep end for some spells (they don't seem to balance in some instance, but I'm still trying to understand all the magic system).