The short answer is that bookkeepers usually charge by the hour, with rates varying depending on the region and the experience of the bookkeeper. However, some bookkeepers will charge a flat rate for their services, regardless of how many hours are required. This flat rate may be a set monthly fee, or it may be a per-project fee. Ultimately, it depends on the individual bookkeeper and what they are comfortable charging.
The amount you can charge also depends on what services are being offered. If a bookkeeper is experienced in handling invoice tracking and payroll, these types of services may have an extra cost. You may want to charge clients by what services they require, or just stick with an hourly rate, depending on the needs of each client and how many clients you have or plan to have.
Some bookkeepers also offer discounts or packages for small businesses. Most business owners are going to do their research and find a qualified, trustworthy bookkeeper who is affordable. When determining what to charge for the services you have to do a little research and consideration yourself. You have to think about many factors, including your experience, knowledge, and reputation. A well-known, experienced bookkeeper will be able to charge more for their services than a person new to the profession just starting out.