Local businesses that advertise on Yelp receive an average of $23,000 in extra revenue

"The Boston Consulting Group, a leading advisor on business strategy, recently surveyed 4,800 business owners to learn the economic impact Yelp has on small businesses. The results are eye opening: small businesses with a free business owner’s account saw an average of $8,000 in annual revenue from Yelp.
Let that sink in for a minute. That’s $8,000 gained just by claiming their free business account on Yelp. Clearly, local businesses are benefiting from a Yelp halo effect that translates into very real profits.
Even more interesting, the survey revealed that Yelp advertisers benefit nearly three times as much, generating average annual revenues from Yelp of more than $23,000.
For certain categories, the returns were even higher. Breaking out the numbers, we saw a number of categories doing exceptionally well advertising to purchase-minded Yelpers:
Home -- $54,000*
Automotive -- $39,000*
Local services -- $36,000*
Hotel and travel -- $36,000*
Shopping -- $24,000*
Health & Medical -- $20,000*
* Average annual revenue from Yelp
Considering that our typical local advertiser spend is $4,200** annually, that’s a pretty good ROI for these savvy businesses."

Source:  Yelp's blog, 19th March 2013