I've added a free listing option for my No CS Degree job board as I've seen a few other indie job boards do this. It's the old freemium technique, I guess.
Ranch Work, for instance, does this very well and they hit $10k in monthly revenue recently.
Free listing: on the website for one week, one time only
Pro listing: on the website for 4 weeks, sent to newsletter and promoted on Twitter
Let's see how it goes!