Just returned from Chiapas after an 18 year lapse. I must say the roads have improved, although you always have to be on guard for unmarked [i]topes[i] - speed bumps -as you enter and exit towns. One invaluable thing I brought with me was a GPS, in my case a Garmin nuvi 2460, about $200 U.S. It has a base map of Mexico which included pretty small towns. The base map was slightly off once in a while, but you could find your general location by zooming out, otherwise I always knew where I was. I didn't spring for Garmin's Mexico map, since I figured it wouldn't have any additional detail in the rural areas where I was going. I also had 20 year old road maps from my last trips that provided me the detail that the GPS lacked. Be on the lookout for turnoffs to roads! What looks like a major road on a map might only be 20 feet wide and easy to miss.
One cost that you have to figure in is for the car rental. For eight days, the insurance only cost an extra $500 plus for a VW Jetta. I understand that the law only considers Mexican insurance if you get into an accident, so you need it! Gas is slightly cheaper than the U.S., at $10.00 pesos a liter for regular unleaded (Magna). The current exchange rate is $12.85 pesos per American dollar, but the hotels usually gave around $11.00 pesos. The Jetta got good mileage, but each fill up ran about $350- $400 pesos, and that was for a little more than half a tank. You never know where the next gas station is, so don't let your tank get much below half, especially in rural areas.
Be careful, Mexican drivers like to drive fast, even on the mountain roads in Chiapas and Tabasco. Just turn on your hazard lights when it is safe and let them pass, so you can take your time looking for streams.
Have a safe trip, and enjoy the scenery. I was surprised by large tracks of palm oil trees in both Chiapas and Tabasco, and selfishly hope that the pace of development slows in this beautiful area.