Water changes alone cost me about $15 in salt, plus whatever rodi costs. Then the electricity has to cost about $20 or more. My fish eat a package of nori in a month, so that's about $6 right there. There's no way this hobby costs me less than $60 per month, and that's months when I don't spend money on corals/additives/equipment/fish. Its got to be over $100 on average if you add that stuff in. I justify it buy telling myself that my car is paid off and I hardly ever drive so I spend no money on gas.
I hardly ever buy fish and I'm nearly maxed out in coral, so it's all maintaining from here....which is still somewhat expensive.