Cheapest way to get from U.S. to Mexico (NO FLYING)

[augentier]

What is the cheapest way to get to Mexico from the U.S.? I don't want to fly because that's expensive and hitch-hiking is NOT on my to-do list.

Is there a bus or train that goes from a city in TX or NM that goes down through Mexico? I'd like to eventually make my way back down to Nicaragua, Costa Rica, or Panama. I figure there are enough buses in Mexico to get me through and to the neighboring countries but what about from United States to MX?