International Data Plans-- I need some help!

So I have an unlocked iPhone and currently use AT&T, but their international data plans are really expensive and I'm trying to avoid using those plans. Im traveling through 6 countries: France, Belgium, Netherlands, Germany, Italy, and Switzerland. I would like to have at least data on my phone, voice service is optional for me.

So, has anyone used Vodafone or Orange in this way? And how much did it cost for X amount of data?