I was determined to get to the airport train by bus, just to see what the minimum cost would be. I did it, but on a Sunday, the buses just didn't run often enough. The wait was long. I wouldn't suggest this as an option unless you were really broke. The better choices are the tourist shuttle or a metered taxi to Phayathai with a switch there to the airport train.
Bus #59 runs from Ratchadamnoen Klang to Phayathai (13 baht). I was standing in the aisle, and my height was oddly a problem. I'm not all that tall, but I'm not all that short either, so I couldn't see out of the window to know where the bus was. I had to keep ducking down so as to not miss my stop.
(You can also take bus #556 to Makkasan, but I'm not sure where you'd catch it, so you'd have to ask at Tourist Information. The other option is to take the canal boat to Pratunam and catch the airport train there, but you'll want to be not carrying a lot of luggage for any of these options as you have lug you bag onto the bus or boat. The canal is the best option when coming from the airport during rush hour. A metered taxi is the best option from the airport on a Sunday or at night.)
I switched to the airport train at Phayathai, and paid 45 baht for the local train to the airport. The express is 90 baht.
So that's it--I believe the minimum price for getting from Khao San to the new airport is 58 baht, and the minimum to the old airport is 23 baht. There might be a local bus that goes all the way to the new airport, but I imagine that would take forever in traffic.
After checking in, I still had plenty of time, so I went to find the legendary, elusive food court that tourists don't know about. It was downstairs near the train level, mostly unmarked and all the way at the far end of the airport. It's a coupon court, so the people handling the food aren't handling the money. Not a lot of atmosphere, and expensive compared to street food in Bangkok, but quite cheap compared to the airport restaurants.
That didn't make me buy anything, as I wanted to get through security before wasting any more time. This turned out to be a mistake as the food past security was much pricier.
I had my last fresh pad thai and mango with sticky rice. Why is it so much better here, I wondered. The best Thai food I've had in New York isn't as good as the worst Thai food I've had on the streets of Bangkok. Why?
I wouldn't know today. I used up the last of my baht and headed to my gate.
For my flight to Singapore. Yes, I had a nutty route home, starting with Bangkok to Singapore.