we just came home from Thailand and went to Bangkok from the airport twice in 2 weeks and found the cheapest way. Outside the terminal there is a big area with the signage PUBLIC TAXIS. Go there and no where else as that is the only area you get the official taxi to Bangkok. It is 400B plus 50B toll charge, others will try to escort you to their taxi, avoid it. You will pay in advance so there is no hassle at the end. Wish I knew this the first time.