The train fare varies by station, so 35 THB for SBIA/BKK - Makkasan, and 45 THB for SBIA/BKK - Phaya Thai (one-way prices).

The first train is at ~ 06:00 and the last train at ~ 00:00. Travel time is ~ 30 minutes.

Not sure exactly what it is you want to do, but you could stay at the Novotel and take the train into the city for the evening then back again before 12 midnight. Or stay at a hotel in the city, using the train into the city, but you'd have to take a taxi back to the airport to make that flight the next day as the first train might be cutting it close.