The $64000 question and often more than that. But it really is not a simple question to answer. You can make it easier by defining what you mean. Working up a marketing strategy or developing a brand, on going consultancy, marketing planning and implementation, copy writing, social media management or ongoing marketing support services will each have different start and end points on the cost scale depending on the complexity of the project. What you are paying for with a good marketing consultancy or agency is expertise, empathy, creativity, transparency and reliability. Their time and your money.
How much of this resource you need over what period determines the size of your investment. It should be seen as an investment too, as you will expect it to provide a return as you would with any investment.
So, what should the scale of this investment be? A much quoted rule of thumb is between 8 to 10% of turnover. This is an average figure and will be higher initially as the costs of setting up requirements pass through and then ease back as the process is more about maintaining ongoing activity. What ever the amounts involved they should be clearly set out by the agency and agreed with the client before proceeding.