This is one of those questions that the answer changes every day, carrier and more. There is no fixed rate world wide rate so go shopping. Here I use my Virgin Mobile 35 buck a month access when I'm on the go. Seems more than good enough. I added Easyt Tether.
Bob
I know that add-on costs greatly increase the basic monthly charge of a cellphone. Does a tablet (like Nexus 7 LTE) have the same add-on monthly costs as a cellphone? Thanks.

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