correlation (or mere coincidence, for that matter) don't imply causation. how do you know whether the discovery of a url eventually occurred through internal linking or the introduction of the sitemap unless you have isolated that factor?
there arent good numbers to go by. google and the other search engines have said quite often that they prefer to discover urls by following links instead of submitting sitemaps. however i have seen sitemaps help large websites increase the amount of pages indexed.
i cant tell you what percentage because as phranque points out i wouldnt know if google found the url via sitemap or via links or possibly even via toolbar usage.
sitemaps can help certain websites with indexing issues. imho most websites are better served by making sure all pages are internally interlinked and developing inbound links from many external websites. this will help indexing and more importantly improve the rankings of the pages that are indexed.
dont forget that unique content can also have a significant impact on your indexing %. if you have 1 million pages and they are all identical, then search engines will not want to index all of your pages. good luck