First you want to think of what sort of business directory you'd like to build. You could build a business directory for local businesses targeting a suburb or city, or you could target a specific country & maybe worldwide if you get enough traffic. You can either target all types of businesses or target a specific niche, it all depends on what you'd like to design your products around.

You need to think of a product that you can offer to businesses that will benefit them. If you can offer a business exposure or traffic to their website for a monthly fee, you're on your way to running a successful directory. Try to think of something unique that will help businesses get something in return for spending a monthly fee with your business.

Offer something free to benefit the visitors of your site & this can really increase traffic to your site & get your branding out there. Some sites offer a free listing while others offer link exchange & various other services. The bottom line is you nee
d to offer your site visitors something of value for visiting your site otherwise it's likely to get minimal traffic.

If you're just starting out then it's always best to charge a low price for your featured listings. I wouldn't charge anymore than $5 per month if I'd only just started getting traffic to the site. You want as many people as possible to join so they can benefit from your product & hopefully tell everyone about it. If you're offering something of good value to a business for a great price, it won't take long for your site to start getting traffic. You could also do PPC (Pay Per Click) to increase traffic to your site.

Designing a directory is time consuming & will have to be updated regularly if it's to be successful. You will need to spend around 12 hours per week managing the site to update content, process orders & approve business listings. But it's well worth it in the end if you can build up enough clients, you will generate income from adwords & also your clients. You could also offer various other services like web design & SEO to generate income.