It was 400 bahts for a taxis from the arrivals - better to exchange only a small amont for the taxis we have arrived in the night before and some exchanges was open you should get about 40ish to the uk pound in there and 49 ish in Bangkok.
Maybe better to do that (pay the 400 bahts) than to go up to the departure floor where everyone is coming in and grabbing a taxis.
to the left side looking at the doors, theres a True booth along the line of boothes you might see Dtac who are also ok but cant remember there being one in there, make sure your phone is open though :)
Not done the train from the airport yet but intend to, dont think it runs in the night, the sky train doesn't run all night either.
Hope you enjoy your holiday, our 9th time in March i dont know what it is about the place but you will love or hate it but do take some time to enjoy it, use the sky train where or if you can, theres also the underground and link to boats on the river so dont think theres just taxis's - hope that helps