For small businesses, common structures cluster around: monthly retainers (a few hundred to several thousand depending on scope and seniority), percentage-of-ad-spend for media management (commonly 10–20%), and project fees for builds. Wide ranges, because 'agency' spans a freelancer to a floor of juniors.
The number that matters more than price is who does the work: senior prices with junior execution is the classic big-agency trade. Ask directly who touches your account and how much of their week you get.
And denominate cost honestly: fees PLUS ad spend PLUS what lock-in would cost you to leave. A cheap retainer inside someone else's accounts is the most expensive option on the menu.