I'm with Bitt on this. Given your minimal usage, prepaid phone contracts can be quite cheap (see, for example, Cricket Wireless, Virgin Mobile, etc.). The major players like AT&T also offer this sort of service (e.g., AT&T's "Go Phone").

I'd recommend completely separating your Mexican phone from your U.S. phone. It seems silly, but it lets you avoid all the silly roaming rules. You simply just get two phones with two plans. You may cleverly try to get phones made by the same vendor, such that they'll share chargers and other accessories. This helps you avoid getting slammed with nasty international roaming charges, but it means that people have to try two different numbers to find you. On the other hand, people calling you never have to worry about the cost of making international calls.

The sort of basic phone that you want is going to be available at minimal cost from pretty much every vendor. You should find you have plenty of options...