The cost depends a lot on the client’s available ad spend, the complexity of the work done, and the pricing structure the agency uses.
For low-budget and low-complexity campaigns, you can expect to pay a flat monthly fee of anywhere from $100 to $2,500.
For marketing campaigns focused on generating leads and sales, with multiple landing pages and target audiences, you will likely pay based on the results generated or your spending.
In either case, agencies usually charge between 10 to 20%. For example, in a performance-based arrangement, if an agency generates 100 sales a month, each one worth $200, you will pay $2,000 (i.e., 10% of $20,000).
In a budget-based arrangement, if you spend $10,000 a month, an agency may charge you $1,000 on top of your budget for managing your AdWords campaigns (i.e., 10% of $10,000).
The benefit of the latter two structures is that the agency is invested in your success. By aligning the incentives, you can expect the agency to perform at its highest potential.