Are professional fees a fixed cost?

Are professional fees, such as those charged by accountants, lawyers, and consultants, inherently a fixed cost, or can they vary depending on the scope of work and the specific services rendered? It's important to understand the nature of these expenses in order to accurately budget and forecast financial performance. While some professional fees may be quoted as a flat rate for a standard service, others may be billed on an hourly basis or as a percentage of the value of the transaction or project. Therefore, it's crucial to thoroughly discuss the terms of engagement and the potential cost implications with the professional service provider before engaging their services.