By some estimates, the average ROI for email marketing is $44 for every $1 spent, when executed correctly. Email marketing amplifies all other activities as users are brought back after an email address is captured via PPC, SEO or social media.
Email marketing technology is inexpensive. Any small business that isn’t actively maintaining and growing at least two email lists (prospects and existing customers) doesn’t have their house in order.
Read more about which channels are most effective for lead generation over on searchenginewatch.com.