It’s usually billed at an hourly rate but can be negotiated on a project by project basis just like web design or SEO.
Really you have two option
1) Write the Copy Yourself – Only advisable if you have previous experience and the time to put into the project. The upside is it can be very lucrative since it’s essentially pure profit except for your time.
2) Hire a Copywriter – If you are doing local marketing it’s highly recommended that you take this route so you can focus on building your business.
If you aren’t in a position to hire a copywriter full time it’s no big deal. There are tons of copywriters that work freelance or project by project.
When you write your proposals or invoice your clients you will want to simply mark up your copywriters rate/fees just like you would with any other outsource or in house employee.