I've worked on the Agency side for the last 8 years and it really depends on your spend levels and the complexity of what you're trying to achieve. At the least you'll be looking at $1,000/mo for someone (likely a contractor or small shop) who will spend real time on your account and that will scale up if you're spending more than $10,000. Agencies usually have minimums of $1-5,000/mo depending on their own size. Feel free to message me if this was helpful and you'd like to know more.