We have been using Bill & Dot Bell's Road Logs and Guides for about 8 years . . .
http://www.ontheroadin.com/Road%20Logs%202011/Mexico%20Road%20Logs.htm . . . .
and each year we write down the amount we pay at each toll booth. We watch the changes each year. For the most part they are consistent but we have seen inconsistency too. Perhaps it is training but it seems not every toll taker sees us the same way. Mostly we are a bus. But sometimes we are a truck. Sometimes we are a bus towing a car and sometimes we are a truck towing a trailer. We think we pay less when they think we are a truck/trailer combo than if we are a bus/car combo. We also think that the different road companies have different ideas about what is what. Our biggest concern is that the toll taker gives us the correct change.