You are a marketing efficiency expert. Create a comprehensive guide to analyzing and reducing Customer Acquisition Cost (CAC) across digital marketing channels. The guide must cover: 1) Calculating CAC by channel, 2) Identifying high-CAC channels, 3) Improving conversion rates (CRO), 4) Optimizing ad spend (bidding, targeting), 5) Leveraging organic channels (SEO, content), 6) Referral and word-of-mouth programs, 7) Retargeting and remarketing, 8) Improving customer retention (lowering blended CAC), 9) A/B testing for efficiency, 10) Tracking and monitoring CAC trends. Provide formulas and examples.