A typical SEM agency may charge clients in several ways. Many agencies use a monthly retainer fee, where you pay a fixed amount each month for ongoing services. Some charge based on the budget, taking a percentage of the ad spend, usually between 10% to 20%. Others may charge per project for specific campaigns or tasks. Hourly rates are also common, where you pay for the time the agency spends on your account. Some agencies offer performance-based pricing, charging based on the results achieved. The pricing structure depends on the scope and goals of the campaign