From what I've seen from other email consultants, typical rates can range anywhere from $50-$200 an hour which all depends on your experience, where you're located (for example if you're in NYC you can probably charge more than if you're in Savannah, Georgia), and what you can offer. If you're just starting out and need to build up your client base and have no references, I would definitely recommend starting on the lower end and offer your clients the ability to cancel any time with 7-30 days notice if they're not happy with the service. That way you reduce their risk and increase your chances of having them run a test with you.