This is akin to asking what is the ROI for usability. Speed is a key component of usability. Small improvements in speed can take critical pages below typical attention thresholds, and dramatically lower bail-out rates and abandoned shopping carts. I talk about this in the book, but compression alone can save 30-50% in size and bandwidth costs. Webmasters who have employed compression and optimization typically save 30 to 50% off their bandwidth costs, and retain more customers, and have improved conversion rates.
The buzz on this book is good, and I look forward to using it to keep PlaybackTime lean and fast as I add functionality. [via Ranchero]