It really depends on your pirating style.
When and if I need to, I can make a lot of money via pirating.
20 million per trader hauling ore is good. You can charge higher/lower depending on how you feel.
I charge 45 million on convoys on the grounds I'll leave them alone.
tl;dr when I'm rping, I charge low (2-5 million) when I need money I ask for what I want (15-20 million).