How do you handle client fee breakdowns in temporary staffing?

My agency has had a temp contract for the past five years, and unfortunately, they pay a very low rate. A few years ago, someone shared a cost breakdown of our fee structure with them, and since then, they’ve had us in a difficult position.

With upcoming changes in tax rates, levies, and wage increases this year, we’ve decided to try to raise our overall charge rate slightly—by about 0.1%. Now, the client is asking for a detailed explanation of how we arrived at that figure.

I’m looking for advice on how to respond to this request. The current charge rate isn’t sustainable for us in the long run, but losing this client isn’t an option either. We typically don’t provide cost breakdowns to our clients because it often leads to complications like this. Any suggestions?