My thought is that a listing on the Yahoo directory is probably good value to help kickstart a new quality site. The main value of the listing is going to be in initially getting the attention of the search engines, etc, focussed on your site. Once your site has matured and is receiving traffic and link juice from a large number of more organic traffic sources, the benefits of a Yahoo listing are likely to be diluted to the point where it may no longer be worth it. The question is, do you have reason to believe that the listing is making you at least $300 in overall value. If so, you may as well keep paying for it.