Study contrasting SEO and Pay per click results included ecommerce sites with a mix of B2B and B2C

Some of the metrics evaluated
• Average order value
• Bounce rate
• Conversion rate
• Value per visitor (sales divided by visit)
• Average time spent on site
• Average number of page views per visit
• Average length of time spent on each page viewed

Average Order Value from paid traffic was $138.04, while AOV for SEO was $117.09.

A visitor from a paid traffic source was worth 41% more than a visitor from an organic listing.

Additionally, paid traffic converted at a 20% higher rate
Study showed that paid traffic returned a more valuable visitor than that from organic listings.

Obviously, needs of individual sites vary, and a variety of factors need to be brought to bear for budget allocation decisions.