In most cases, offering free content is essential - that's why we talk a lot about content marketing here on Learning Revolution. Publishing content is one of the best ways to demonstrate your expertise and establish your authority. Done right, it also significantly increases your chances of being found when people search Google or other search engines on topics related to your courses.
It may sound counter intuitive, but I actually recommend giving away some of your best content for free. It's by doing this that you will really stand out and convince people that they really need to invest in doing more with you.