I've previously written about my basic approach of figuring out how much a No.1 ranking in Google is worth. Aaron over at Seobook went probably a little overboard when he put his 15 page(!) "How much is a Top Google Ranking worth" posting together. However I think this is one of the best SEO postings out there, but you have to be ready to put massive efforts into it by using all his data sources and techniques.
Overall if you are serious about SEO, his approach is really cool even he forgot to include the Wiki tracker, which in my eyes gives great accuracy on volume of traffic on a specific keyword.
Just because of the simple reason that Wikipedia ranks mostly in the top 3 for big keywords.