How much to charge for services?

I'm not sure if this is the appropriate sub because this is more for business owners but I'd appreciate hearing from devs, so here goes. We're a relatively small bespoke development firm. Our business is growing and we're busy. We have BE/FE, PM and QA roles in-house. My question is how do you determine the rates you charge your customers for each of these roles? I know there are a ton of factors that influence how much you charge, like if you're in a major city or not, what part of the world you're in, the market(s) you serve, etc. So to be clear; I'm not asking WHAT you charge, but how you determine HOW MUCH you charge? I hope that makes sense.