I'm starting to add trade routes to my empire but the trade route never gets beyond 150 dust per turn and maybe a fraction of the luxury resource. Even when I have a system of 12+ inhabitants and have placed both the trade hq and the subsidiary i still dont see much income. adding more ships doesn't seem to have an effect. What am I doing wrong?

The income is based on distance between the subsidiaries, how many you have, and the value of those systems. You generally want to build subsidiaries at the furthest out systems in your empire which are big.

AFAIR trade value of systems in the path also matters.