I'm curious how many regular links account for a deep link? What is the difference really for SEO purposes? Would I be better off submitting deep links or regular links to these directories? Should I pay for a service that will submit deep links for me?
It is not like that if you are getting one deep link means you are getting value equivalent to 2, 3 or 4 regular link. For SEO purpose it is good to get links for inner pages so that SE can believe that your website's each page is getting back links. Most of general directories does not support deep links, but paid directories do. You can pay for the service and if you are thinking of using this type of service then you can contact me too.
DeepLinks are useful since they allow the inner pages to be get some search engine and pagerank love. Moreover promoting your inner pages also helps you get it outta supplementary results and helps for better ranking. Inshort good for SEO.
Deep links are pretty important, especially for large sites. Just as backlinks help your offpage SEO for your homepage, deep links help to get your inner pages ranked as well. For large sites, deep links also help to get more of your pages indexed. I would certainly submit deep links to directories that offer them. Most of the time these require paid submissions, so be selective. Check the directory list in my signature. There are about 20 free web directories in the list that offer free deep links (multiple URL submissions). They are marked with an asterik next to their name. You can use social bookmarking to get some deep links and traffic as well. While the cost is free, signing up and submitting takes time, but it is worth it. If you write and submit articles, you can also get deep links.
Sitelinks aren't awarded to the pages brought link for. They are awarded to the pages with highest searches, traffic & popularity.
Directories (general) typically don't get many natural backlinks unlike a blog or forum may do, so deep links are a great way of providing the inner pages with some inbound links.