G Suite starts at $6/user/month. They also offer business and enterprise plans that are $12 and $30/user/month as well.
To get signed up and going, click on the “Get Started” button under the plan that the client wants to go with. From there, just follow the prompts. They should be fairly straight forward. It will also have you update MX records for your domain. If the domain is on a system they support it is very likely it can auto configure the MX records for you. If not, you will need to manually update them yourself once that time comes.