The amount of money is (tradermoney - ( pirating = piratemoney ).
Your math fails on many levels:
1. You have an equality inside parentheses? What the hell is that for an equation?
2. You make a totally unfounded assumption that the number of pirates equals the number of traders.
3. It's actually possible, even with a 1:1 pirate:trader ratio, for pirate income to not only equal but exceed trading income.
Your actual equation should be:
t = T - p
where T is the potential max income of the trader, t is the actual income of the trader, and p is the actual income of the pirate. p could certainly be larger than t, it just can't be larger than T.
For example:
Timmy Trader trades tin for 10,000 credits per trip.
On average, every trip Peter Pirate plunders 6,000 credits from poor Timmy Trader.
Timmy's actual income is 4,000 credits, and Peter's is 6,000, which is 50% greater than Timmy's.