Posting on Craigslist is free in some categories. For example, if you want to sell your grandmother's china, the post is free to put up on Craigslist. If you want to rent your Boston-based apartment, it will cost you $5 to put up an ad. And if you are posting about jobs, it could cost you anywhere from $10 to $75. In most cases, Craigslist only charges for ads posted by auto dealers or realtors in commercial real estate and those posting about short-term gigs and job opportunities.