I really enjoy my tank, but this is an expensive and time consuming hobby (at least at first), and it is definitely not a hobby to be undertaken on a whim after watching Finding Nemo or visiting the LFS (not saying that's you, but if it is, think twice). I'd say it has cost $600 - $800 (including livestock, supplements, test kits, etc.) to get my 30G tank up and running. I still need to spend another $100 - $200 on better lighting so I can keep some corals and such (and then I'll have to spend more money for the corals, they get you coming and going in this hobby). Don't forget the time cost either, both for working on the tank and doing proper research beforehand.