You realize and self-publish your comic book or graphic novel. A lot fo time and effort. You deserve a few money, at least to cover printing costs! While this way of thinking seems legit, the "power of free" can really boost your popularity and -yes- sales.
The power of free
Someone once told me comics are better promoted like drugs: first taste is always free.
After all, how can we expect to attract new readers/buyers with an awesome cover image alone? These times are hard, people have less money than ever before and wants 110% guarantee their purchase is worth.
You have to show your stuff in order to win their fears, let them enjoy the characters and grow fond of them. By giving away free stuff you're actually rising the value of your work, because more and more people will love and ask for it.
There are basically two ways you can offer free content. I suggest you to use them both because they proved to be effective.
Digital free gifts on a regular basis
You can produce free comic strips and side stories to gift your audience. Share it on a regular basis and offer something new -let's say- every week. The key here is to be regular. Yes, you're a busy comic author, but this practice will rise the awareness about your comic, so it's well worth the extra effort.
Contests and sweepstakes
Contest are another great way to put a spotlight on your comic books. Hold a simple sweepstake and draw 1-3 winners per month. Giving away 1-3 free copies per month won't kill your business. You'll gain happy fans who'll tell everyone that they won your contest and how much they're happy about it.
Side note: be very careful to hold a contest or sweepstake on Facebook. its policy is very strict and your page/profile could be shut down without warning! Here's a good example about how to hold a sweepstake on Facebook.
If you use the "power of free" correctly and within your possibilities, you'll gain in user-experience, growing audience, buzz and awareness about your comic book.