You can get that in Europe, but you usually need to sign up for a contract which usually means having an address and a credit record. Can a tourist visiting the US can get your $50 a month plan?
I just checked and Three UK actually has a £37/month plan (so roughly $50 at this time) with unlimited data, calls and texts, which includes data in 71 countries, and you only need to sign up for a 1 month contract. (It's £32 if you have a UK bank account.) I don't know whether a visitor can just walk into a store and sign up for it though.
This would be something I would be interested in. What company, or plan is this called? Is it possible to get link or a name so that I can actually look up the info? This might be exactly what I need. Thanks.