While your setup costs are relatively low, you need to think about how you're going to promote and drive traffic to the site... Advertising can cost a lot of money.
Do you have some other websites already built up with high page rank that you can use to boost the rank of your new directory? That's a good starting point.
If not you may need to build more than just a directory site - build up PHP resources, information and create good content to speed up the growth of the site. These days just creating another stock standard directory isn't going to cut it unless you have some way of driving large volumes of traffic. Personally, I only think it's worth it if you are passionate about PHP.