[whatcaneyedo]

It only costs as much as you want to spend. If you dont have a phosphate problem or hair algae problem you probably dont need to run GFO. Carbon is basically the same. If you already have clear well skimmed water and you dont mix a lot of species together that conduct chemical warfare with one another then you dont 'need' carbon. Its entirely possible to run a successful problem free tank without either.

But if you're like the rest of us with a mixed reef aquarium that you frequently overfeed... it might cost hundreds a year.