How do you handle client fee breakdowns in temporary staffing?

Our agency has been under a temp contract for five years, and the client pays us a very low rate. A while back, someone shared a detailed cost breakdown with them, which they’ve used to hold us to that rate ever since.

With new tax rates, levies, and wage increases set to take effect this year, we thought it was a good opportunity to adjust our overall charge rate slightly. We’re looking at only a 0.1% increase, but now the client is asking for a detailed explanation of how we arrived at that figure.

Any advice on how I should respond? The current rate isn’t sustainable for us in the long run, but losing this client is not an option. We usually avoid providing a cost breakdown to clients since it often leads to complications like this.