Satmetrix has just released a study on the value of Word of Mouth marketing based upon their Net Promoter Score system.

They put a $ value on what extra revenue a positive promoter brings versus lost business for negative promotion.

To quote from their press release

"....Word-of-mouth is one of the most powerful and trusted marketing sources, but measuring and more importantly, understanding its financial impact hasn't exactly been clear cut,' said Richard Owen, chief executive officer at Satmetrix. 'Our study does that by examining customer behaviors and quantifying them in the larger economic picture that links loyalty with growth....''