I'd suggest somewhere between 5-7 dollars for featured links to start off. That way it looks like you are serious about your directory, but you are not charging more than people can afford. Make sure you use search engine friendly URL's And make sure you give featured links an extra benefit. SAy a different backgorund color, alexa thumbnails for featured onlys, a animated graphic next to them. ANd make sure you get plenty of backlinks
I agree the featured listings should get some benifits like the ones you mentioned. When I turn on my SEO friendly URLs I get an error, not right when I change it, but when I go to a category in the dir. Is there something I am doing wrong?