Ask yourself why a business would pay to be listed in a directory that has very little or no distribution (by distribution I mostly mean exposure/web traffic)? When it comes to content business, with some experience, I can tell that you that success doesn't come overnight. You really have to plan for a long-term ride, build your directory at first, establish some kind of presence, before you can charge a fee.
As far as reaching your clients, why not simply contact them directly, introduce yourself and offer them a free listing? That way you are building a relationship from the start, doesn't matter if it is costing you or not, you have very little to lose. Maybe once the directory grows, you can substitute free listings with paid ones and at that point some of the free clients will be ready to stay with you by paying the fee.
You should ditch the fee for some time, until you build some reputation. Some satisfied clients of yours could write a good review about your business. Maybe this should be the way for them to pay in the beginning.