An expense represents the cost of doing consulting business. Common business expenses include wages, salaries, bonuses, equipment, software, materials, travel, etc. The significant costs in consulting business are the salaries of consultants.
In general, any agency expense consists of compensation of consultants and overhead costs (cannot be directly related to products or services offered, like rent, advertising, and administrative staff salary).
Detailed expenses calculation in a consulting business is a complex subject (see articles by Jason Cohen or Hyam Singer).
A consulting firm has 6 consultants, each of whom costs $170,000 per year. There are 3 employees responsible for marketing, sales, and finances, which brings the total annual overhead costs to $450,000. The annual expense of the firm is $1,470,000.