I also tried the Sales on Commission when I first started a website directory. It didn't work, so I changed tactics.
I think you should offer a Free Listing to businesses.
Most business directories offer free listings that are simply a Business Name, Location, Phone and or a website link.

When I started my directory for Eco-conscious products and services ( www.findagreenstore.com ). I first offered free listings. after I had 60 free listings and my site traffic increased, other businesses started to buy ads. As of now, text links are free, but banner ads and product placements are paid for. I also keep my prices low while I grow traffic to the site.

You can do 2 other things.
1) lower your prices and keep them low for the next 6 months or until your site has more traffic. or
2) Have an Ad Listing Sale of 50% off the regular price. advertise in your local classifieds, and email local businesses directly.

Think of new ways for people to advertise on your site, such as product placements or commercial videos. There are hundreds of business directories already so you really need to stand out.

If you are on Linked In or Facebook, join some local business groups and get involved in discussions.

Good Luck Leah Oviedo
Mercado del Mundo