How does internet data get charged on the iPhone?

So I just want to get a general idea of how much data I expect to use on my iPhone. I have Wi-Fi at home and I'll rarely need 3g outside of home. Don't plan on doing any youtube or skype, just gonna check facebook, email and ebay mostly. How much mb of data is this per month?

Depends on your carrier. I have AT&T and I get 3GB a month. Just download Data Usage from the App Store and it will tell you how much data you use a month. I would recommend 3 though because it doesn't cost much but you still get a large chunk of data to use