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The benefit of a Yahoo listing is diminishing and a site that has 20,000 members could probably do much better with a competition.
Why not offer a $300 prize draw which only needs a member to invite a friend to get in the draw, run it for a month or two before drawing it and hey presto, many new members!
Repeat at will
Google (And Yahoo! seems not to care much about it too.) doesn't value Yahoo! directory listing like it used to. Even if Google still looks highly of a listing in Yahoo!, you could get a listing in many other directories that require no payment for submission and with all those directories together would beat out a Yahoo! listing. You also have to use your site's title in the listing, which kills the reason of submitting even more IF you are doing it for search engine ranking.