How do you handle client fee breakdown requests in the temping sector?

My agency has had a temp contract for five years, and we’ve been stuck with a very low rate since a past client received a detailed breakdown of our fee calculation. As a result, they know our costs and have leveraged that information against us.

With upcoming changes in tax rates, new levies, and wage increases, we decided to adjust our overall charge rate slightly—around a 0.1% increase. However, the client has now requested a detailed breakdown justifying this change.

I’m looking for suggestions on how to respond. The existing charge rate is not viable long-term, but we can’t afford to lose this contract. We typically don’t provide cost breakdowns to clients for this very reason.