From the US? It is international travel. If you mean NEW Mexico, that is domestic travel, though.
No.
Mexico borders the southern United States, so minimum distance/time goes from 0 (say Laredo, TX to Nuevo Laredo, Mexico) up to several thousand miles (say, Hawaii to Cancun) and several hours worth of travel time.
Yes.
If in Mexico travel north. There is a border crossing. Pass that and you are in the US
As long as you have an American passport, you can enter Mexico. Problem is on your way back to the US. Without a valid proof of citizenship, you will not be abe to return to US soil.
ye you do
Yes, provided you have an American passport.
Yes, they do.
Yes.
Yes. Most of the time the US has a trade deficit with Mexico.
A great website that tells you the best way and time to travel to Baja, Mexico on a budget would be at gomexico.about.com/od/.../f/when_to_go_mexico.htm. This website gives you great detail on which way and the best time to travel to Baja, Mexico with a dog on a budget.