Yeah 10 mil is too much, even with a rank of 91. If the guy just spent a week straight grinding cash to buy a BS for his approved faction, that doesn't mean pirates should expect to take a bigger chunk.
Look at his ship, look at his cargo, act accordingly. I've taxed as much as 4 mil and as little as 100 units of cargo he was carrying. I don't try to make a real profit off of my prey, that just makes them more likely to run and less likely to RP the encounter.
"To gain a hundred victories in a hundred battles is not the highest excellence;
to subjugate the enemy's army without doing battle is the highest of excellence."