What do professional bookkeeping firms charge?

I am a career accounting professional who is thinking about starting my own bookkeeping firm in Southern California. I don't know if bookkeeping firms charge their clients by the hour or if they instead charge a flat monthly rate. We will do all facets of bookkeeping except for preparation of income taxes. If professional bookkeeping firms charge by the hour, then how much? If they charge a flat rate for full service bookkeping, then how much?