First, before you buy a coral, make sure your nitrates, nitrites, ammonia, and phosphates are at 0. The amount you will spend on a coral truely depends on what you want and your spending limit. Common corals like xenia, some leathers, some button polyps, and some mushrooms can be as low as $5 per frag, sometimes even less. I could help you out with a couple frags that are very hardy and great beginner corals for really cheap. There are also corals, like the infamous PPE's(purple people eater zoanthids) that go for 25-75 per polyp depending on where you get it and how nice the person selling it is and the green polyped sarcophyton(toadstool) that can go for 25-45 per inch frag. Color, size rarity all play a role.