Is a plateau higher or lower than a coastal plain?

The answer is - The plateau is higher than the coastal plain.

The plateaus are highlands, on occasions higher than some mountains. They are predominantly flat or wavy, so they look like big plains on higher altitude. 
The coastal plains on the other side, are very low, close in height as the sea level itself, which actually makes them the second lowest plains after the depressions, so they are much lower than the average plateau.