For those, who want the cheapest way to travel to the city or the bus station train station etc, the absolute cheapest way is to take free shuttle bus to the Airport Transportation Centre. Buses fan out to various destinations in the city, except the the Northern Bus Terminal in Morchit, but even to Don Muang Airport for 35 Baht. They are supposed to run round the clock. For Morchit, I found a bus for 30 Bahts. The cab fare from the airport to the Bus terminal cost me about 400 Baht, including the toll.