There are basically 2 types of highways in Mexico, Cuotas (toll roads), and Libres (free roads). Toll roads are not particulary cheap. You pay by the axle and a dual rear wheeled vehicle, for example, counts as 2 axles. However, toll roads are to a large extent, up to US interstate standards. Your chance of having an accident on one is far reduced, and you will not find yourself crawling behind a sugar cane truck doing 30 km per hour. Furthermore, you avoid small towns that may contain hazards like low hanging trees and archways. Since many toll highway sections are privately run they can vary in cost & services. Some actually provide insurance against things like broken windshields. To give you some idea of costs, an average RV will pay about $250 in tolls to go from Nogales to Puerto Vallarta. On our caravans, we utilize toll highways as much as possible although we will take some of the better free roads.