Assumptions used to build the model:
Total ad spending by SMBs ⇒ average of these two $5.4 billion in 2010, according to this article  from BIA/Kelsey.
$2,300 * # of SMBs in 2011. This survey estimates online ad spend at $2,300 per SMB in 2011.
Share of ad spending on Google AdWords ~29%. This is a product of the numbers below. By 2010, 37% of online ad spending will go to paid search. 
Google owns 80% of ad spend on paid search. 
37% * 80% = 29.6%
Google multiplier = 8.  We conservatively estimate that for every $1 a business spends on AdWords, it receives $8 in revenue through Google Search and AdWords. Thus, to derive the economic value received by advertisers, we multiply our AdWords revenue on Google.com search results in 2011—what advertisers spent—by 8.
Model '(Total online ad spending by SMBs) $5.4 billion * (share of ad spending on Google AdWords) ⇒ 37% (online search) * 80% (Google share) * 8 (this is the Google multiplier: $1 of ad spending ⇒ $8 revenue) = Economic impact of Google AdWords = $12.8 billion
(# of SMBs without a website ) 1,590,000 * ( EconImp_Goog_SMB ...