These days the tax should be included in your ticket, so no need to worry about dealing with it on your actual departure. As noted above it only applies to pax who have entered Thailand. So if you are on an International-International transit you would not need to pay (as long as you stay airside and do not formally enter the country).

I had a case a couple of years ago where for some reason Aeroplan did not include it in the award I booked through them. When I checked in the TG F staff informed me that I needed to pay. As noted by some one else up-thread they seemed a bit embarrassed by this. I was not happy but luckily had enough cash in my pocket.

Unless it has changed recently it is 700 baht.