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It doesn't cost too much to maintain the tank - cost of water and salt, or if you prefer to buy it premixed, the cost of saltwater, averaging about 5 gallons/week, and the cost of carbon or other filter media that must be replaced.
A much bigger cost might be the cost of food, if you're thinking of getting a squid or octopus.
not much as for maintanence...food is what you need to worry about, thankfully most of us have a local fish supermarket where we can find fresh and inexpensive seafood for our ceph's, try finding one near you.