We use a mixture of hired actors and volunteers for staged training events. The rate we pay is £150 per day for the professionals and £100 for the volunteers. (We don't pay anyone whose NHS employer is also paying them for the day). We get professionals by asking local theatre groups.
We do voice over in house. The important thing is clarity of voice and quality of recording. An understandable accent is a plus too. The advice I can give is this: get people to practice, get people used to their own voice and get a Blue Snowball mic. They're lovely :)