Cheap for both short and long distances.
Because a private car for most Thais only exists in their dreams, for the long and less long distances most of them make use of Public busses.
There is a well organized system with many different kinds of busses. There is hardly any village not connected with one or another mode of transportation.
For the long distance busses Viengtravel can make reservations and provides tickets.
For the short distances, (50 to 100 km) apart from city- and outer town busses, there is also the so called ‘Song Thaeo’. This is a pick-up truck with in the back on both sides a small bank and place for 8 to 12 people. The color of the car tells you what direction it goes.
In Chiang Mai the red Song Thaeo’s also functions as a bus without a particular route for a fixed price and taking other passengers on as well. Tell your destination and the driver let you know if he goes there. If you like to use it as a cab, you have to negotiate about the price.