I have about 15 yahoo directory listings in a wide variety of sectors. The direct traffic is minimal - the only way to justify the price is that it is one of the few directories that google still considers an authority site. It can give a good boost to your rankings if you have a well balanced backlink program - but it isn't a miracle worker. For instance, I have a site that gets about 15K uniques a month - all from natural SERPS. It has a yahoo listing is a top level directory category, yet only received 20 visitors last month directly from the Yahoo Directory - Obviously not worth the annual listing fee based on traffic, but it is certainly worth it if you are already in the Top 10 or close to it for your targeted keywords and want to move into the the top 5. (of course, results will vary) If you aren't close to ranking in the top 20 for your keywords, I would probably work on link building before adding a listing if $300 is a good portion of your promotion budget.
I noticed some competitors, when I check backlinks, have links in many different yahoo directories. If I submit my link for 299$, will it just be on the main dir.yahoo or will it spread?
Yahoo gets to determine if they list it in more than one category and whether it belongs in other country listings. For backlink purposes, I don't think google counts any more than one or two anyway - and multiple listings won't pass more PR (if that is important to you). A Yahoo listing is also not guaranteed with payment. Make sure your site is one that it looks like they will list or you will be out $299 - no refunds.