It depends on who you hire and how much you spend, so ask for the fee in writing before you start. Agencies and freelancers commonly charge a flat monthly fee, a percentage of your ad spend, or an hourly rate, and your ad spend itself is paid to Google on top.
What are the usual ways an agency charges?
A flat monthly fee, a percentage of ad spend and an hourly rate are the common three. A percentage fee grows when you spend more, so ask whether the manager is paid more for raising your budget.
What should I ask before hiring an agency or freelancer?
Ask who owns the Google Ads account, what you will see in reports and how long the contract lasts. Be wary of anyone who guarantees a top ad position, because no one outside Google controls where your ad appears.
Does a Google Partner badge mean they are good?
It means Google has certified the company to manage Google Ads accounts, which is useful but not a guarantee of results for your business. Partners are not Google employees.
Source: Google Ads Help: Third-party agencies and Google Ads
Is there a cheaper way to get the work done?
You can do the routine checks yourself or use software that does them and asks before changing anything. Whatever you choose, make sure the account stays in your name.