If you travel internationally, chances are you've been bitten by the cold, cruel fangs of international data roaming. Carriers make deals with other carriers to use each other's networks, and not matter what they work out, we end up paying for it. A lot. Phil Nickinson has done some digging into U.S. international data roaming rates,and this is what he's figured out. From Android Central:

If you just have to have roaming data, though — and there's nothing wrong with it so long as you're willing to pay up — it's important to do the math. Sprint and T-Mobile are significantly more expensive than AT&T and Verizon. Not that you'll likely have much of a choice when it comes to switching carriers just for overseas use, but comparisons are good.

Also, almost no carriers have LTE roaming agreements in place, so at best you'll be paying through the nose for slower speeds. If you're planning a trip, check out what the differences are, foreign and domestic, and remember - if you have an unlocked phone, you can usually save a ton by getting and using a PAYG SIM card when you land.

Source: Android Central