My 100g that only has two Banggai cardinals (which I got for free), a single rock with zoanthids, and a cleanup crew has cost me upwards of $2,000 so far. That's been spread out over several months, but it's not a cheap hobby. You can definitely set up a tank for less, but my (not-too-experienced) opinion is that saving money on the setup is likely to cost money and/or gray hair in the long run.