That's going to depend entirely on your carrier (AT&T, Verizon, Sprint, T-Mobile, etc). I have AT&T. AT&T offers a 200MB data plan for $15/month, for which overages are billed at $15/200MB - meaning that if you go over your allotted 200MB, they charge you an additional $15, and give you an additional 200MB to use. They don't charge in any smaller increments than that. They also have a 2GB plan for $25. For that, I'm not sure if overages are billed in 2GB/$25 increments or $15/200MB increments. Either way, unless you're constantly streaming movies or other things that require a large amount of data, I think you'd have a hard time going over 2GB.
They also have a WiFi/tethered plan, with which you can make your phone a WiFi hotspot and let computers around you log in to it or connect your phone and your computer with a cord and let your computer use your phone's internet that way - for around $40 or $45/month, but I've never used that one, so I'm not 100% certain what it's for, how it works or what its limitations are.
Also, anytime that you're connected to WiFi, data usage is free, so if you're sitting in a Starbucks or something and you do want to stream a movie or download music or just get on the internet, it won't use up any of your data.
Unfortunately I have no idea what the other carriers' plans are like. I know that some of them offer unlimited data (Sprint, maybe?), so that might be worth looking into. Really it's going to depend on what you plan to do with your phone. I have the 2GB data plan from AT&T, and I download apps fairly often and check email and go on websites very often, and I've never had an overage, and I'm rarely connected to WiFI. My brother also has the 2GB and spent the first month he had his phone watching Netflix on it, so he almost went over, but they send you emails/text messages when you're nearing your data usage limit, so you can know ahead of time and slow down. You also can check it anytime you want by dialing a certain number on your phone or going online, so it's pretty easy to stay within whatever limits you set for yourself.
Hope that long, disorganized ramble was helpful 🙂 Merry Christmas!