Google loses an estimated $1bn a year in the US through consumers' use of ad blockers

"Ad blocking remains a controversial topic. Based on a variety of data, one company attempts to estimate the financial loss to Google. Of the estimated $41 billion in revenue Google reported in 2014, PageFair estimates that $17.6 billion came from the U.S. Some $1.9 billion represents the estimated U.S. revenue available to Google on google.com that the company did not generate as a result of the 10% of visitors having ad-blocking technology installed in their browser.
Google recovered about $942 million -- a portion of the $1.9 billion in the U.S -- by being white listed, estimates Sean Blanchfield, technology entrepreneur and CEO at PageFair, which wants to "save the free Web" by allowing publishers to serve ads blocked by browser plug-ins like Adblock Plus from Eyeo. Adblock allows people to block annoying ads, while some "acceptable ads" from Google and Microsoft filter through. The industry calls being "white listed," a hush-hush practice that insiders say requires the engines to pay a fee. The German media estimates the fee at $25 million, he said.
Ad blocking also costs brands billions in lost revenue because they lose opportunities to connect with consumers. Blanchfield believes that Google cofounder Larry Page glossed over a critical question during the company's shareholder meeting last week when an investor asked how ad-blocking technology will influence the company's main revenue stream. Page chocked it up to the industry's need to make more "useful" ads."

Source:  MediaPost, 9th June 2015
Update - Estimated to be $6.6bn globally