"Experienced Welsh Guy" is spot on. I suspect if you invested $100 in a trial run you'd find the ROI (return on investment) would be very low for a directory. But, if you give it a try please report back.
I think you're better off choosing some of the sites with heavy foot traffic from Webmasters looking for directories and start buying some links and listings. And don't forget that your listing, your link or your ad needs to be seen an average of 7 times before someone will really start to notice it. Once you're visible out there you'll start to pick up some listings because people know you're growing your Web presence and they will want to be in front of all of the future visitors.
It's scary spending, spending, and then spending more before receiving any PayPal payments, but it does happen over time. (I hope!!!

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Cheers