Media bags in general will be the cheapest way to do Carbon/GFO/Etc. if you are OK with running it passively (not in a reactor). What you are talking about sounds specifically like ChemiPure Elite which is just comprised of mid grade (not that great) carbon, GFO, and an ion exchange resin that doesn't do anything in saltwater (similar to DI resin). It would be far more cost effective, and probably function better to go get a nice media bag, some of the $10/3lb high grade pelletized carbon from [MENTION=2853]New Fish[/MENTION] (or your favorite vendor), and some high capacity GFO.
At this point I just go get Aquaclear 70 carbon bags (3/$8) which last me 2-3 months and run biopellets. I have zero hair algae and only get diatoms when I don't do a water change for 1+ months.