So far that seems right; you could take those steps for travel. I would assume there are escalators at those stations (out of the airport link, into the MRT) but there might not be; there isn't everywhere. The timing isn't wrong but since you will wait on the first train and take time to transit it will take longer than a half hour; how much depends on the luck of the airport link timing, which should run about every half hour.
The cost would be around 100 baht for the airport link (I forget the exact price based on when I last took it), and the MRT is around 25 to 40, depending on distance. A taxi from the airport should cost around 250 to 300, with the surcharge included, so you save money, and probably a little bit of time too, but most people aren't interested in saving 100 baht and 15 minutes to do the extra work. I'd take the two trains but that's me. For other transportation information this site compiles a lot:
http://www.transitbangkok.com/