has hitched in mexico and has written extensively about it. you might try reading her story on it, broken into 8 pieces, which begins here
my own hitching experience in mexico is limited to a few days in the yucatan and the bits of quintana roo near belize. in most parts, there simply wasnt very much traffic even on the largest highways, and things moved slowly. but the weather was great, i enjoyed the company of the friend i was with, and there was no rush at all. an upside was that buses came along the same roads and stopped anywhere along the way, so if you decided you'd rather get somewhere than wait for a lift, you could always flag it down.
people were friendly and i never had trouble with the police. one police truck took us 11 km.
i found costs in mexico to be, on average, about 1/4th of the cost of things in the states.