Google Webmaster Tools is one of the most useful for Website. The major important section are :
Links - See your website back links (internal and external)
Crawl Errors - Shows the errors Google bot found in your website.
Sitemap - See number of Url index by Google.
Robots.txt - Instruction...
It's true PPC is most expensive but in my knowledge PPC is not a part of SEO.
The most expensive and hard seo technique is Guest Posting and get back link from niche website.