I am a US citizen in Mexico on a tourist visa.  I want to buy a house
in LLera de Canales , a small town in Tamaulipas.  I know there are
restrictions against buying within certain distances of a border or
the ocean but this is not in a restricted area.  Someone told me that
at one time in the past, it was necessary for a US citizen to purchase
a permit from the Mexican government to buy real estate in Mexico. 
The government valuation on this property is 260,000 pesos, which is a
highly inflated value--100,000 to 150,000 pesos is more realistic.  My
question is:  Is it necessary for a US citizen to purchase a permit to
buy a Mexican residence?  If so, what is the cost and what is it based
on? Are there other problems I'm apt to be unaware of?

Keep in mind that property rights are not pieces of papers like
titles, but instead are something that only really exists in the minds
of the people.
Property rights exist only where they are respected and/or enforced by
others. This may vary from place to place in a country like Mexico.

If you come back to the US for a while, and you find squatters on your
property when you return to Mexico, the question might not be whether
they are illegal.  The question is: will anyone do anything about it?