How much does it cost if you go over your data plan on Iphone?

I just got a text saying that I was close to going over my data plan for my phone. If I do go over, is it going to be crazy expensive, or just like an extra 20$ or so? I'm paranoid to do anything that uses data right now because I hear these horror story's of people getting 500$ phone bills from data usage.also, if there a way to find out how much you used?

Of course that depends on which carrier you are with & plan you are on.