I've never submitted to Yahoo and we're getting good SERP listings. I don't think it requiers a submission, just a few backlinks to get started in the SERPs.
Now, you don't paid 300 for Yahoo Dir, only 60$ and after 15 days, your sites will be listed! See my signature!
Getting listed is essentially a matter of having backlinks to you that are listed by yahoo as mentioned already, submitting to the yahoo's urlsubmit page, and just playing the waiting game. I think as long as your site is creating relevant new data, your site will move up in the yahoo rankings also which is a major plus.
One thing that might help with getting listed better with Yahoo search is signing up and logging into Yahoo's version of Google Sitemaps (aka Webmaster Central): https://siteexplorer.search.yahoo.com/ Once you authenticate a site you can add a feed similar to your Google sitemap and drill deep into your site to find exactly who is linking to your pages, when the last time they were crawled etc. BTW, for those who don't know Google's Webmaster Central/Sitemaps can be found at: http://www.google.com/webmasters/
That thing needs some work if they want to be able to compare to Google Sitemaps. All it tells me is the last time a page was crawled with either a date and time or unkown.
It could definetely use a bit more of a specific nature. Google is providing a lot more relevant information.
12-15 yahoo backlinks for a year + slight ranking boost; I think its expensive because its not worth it till the last dollar and theres no discount in it.
i find that yahoo takes ages to get you indexed i am getting more traffic from msn than i am getting from yahoo.
Just get some links from sites already in the yahoo directory to your site, and you should be indexed fairly soon..