North American graphic designers vary in the way they charge for their services. Some charge per hour, some per project, or both depending on the client or the type of project. The hourly wage for graphic design services can range from 20$/hour for a new graduate to 300$/hour for an expert. Keep in mind in most cases cheap is expensive. Someone that just came out of school can take 80 hours to do the same job as an expert graphic designer can do in 1 hour. A graphic designer out of school can be very talented, but will not have the experience and know-how to make a design speak to it's audience like an expert would.
How do you know you're getting the best graphic designer at a fare rate? Simple, if you like the designers portfolio, the rate falls into your budget, you're getting good service and the design work is adding value to your product or service don't hesitate, get your design work done and start selling!