Of course, there’s no absolute answer to this question as many search marketing agencies will charge for their PPC services at different rates.
With Koozai, we work with our clients typically by charging an initial fee to set up their Google AdWords PPC campaign or to optimise an existing campaign. This varies based upon complexity and/or number of products or services but is flexible enough to accommodate most budgets.
We then work on a monthly retainer basis, the purpose of which is to allow us sufficient time to further optimise the account, develop additional Ad Groups and provide comprehensive reporting. Given that we see what we do as a partnership with our clients, no one is tied into any long term contracts and we only want our clients to stay with us if they feel we are adding value.
Some agencies will also charge a monthly service fee as a percentage of the spend they are managing through the pay per click search engine(s) and you should check the details if that is the case as this could develop into a hefty additional charge.
When shopping around for a company to use for PPC management services, make sure you have a good chat with them to understand their approach and also ask for testimonials or case studies. If you find a good one, stick with them! Once you do this, the question will change to “How Much is PPC Management Gaining Me?” 
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Koozai provide AdWords management tailored to fit your needs.
What do you think?