Farhad Manjoo writes about the effectiveness of text messaging in political campaigning:
Political scientists have run dozens of such studies during the past few years, and the work has led to what you might call the central tenet of voter mobilization: Personal appeals work better than impersonal ones. Having campaign volunteers visit voters door-to-door is the “gold standard” of voter mobilization efforts, Green and Gerber write. On average, the tactic produces one vote for every 14 people contacted. The next-most-effective way to reach voters is to have live, human volunteers call them on the phone to chat: This tactic produces one new vote for every 38 people contacted. Other efforts are nearly worthless. Paying human telemarketers to call voters produces one vote for every 180 people contacted. Sending people nonpartisan get-out-the-vote mailers will yield one vote per 200 contacts. (A partisan mailer is even less effective.)
These findings create an obvious difficulty for campaigns: It’s expensive and time-consuming to run the kind of personal mobilization efforts that science shows work best. Green and Gerber estimate that a door-canvassing operation costs $16 per hour, with six voters contacted each hour; if you convince one of every 14 voters you canvass, you’re paying $29 for each new voter. A volunteer phone bank operation will run you even more—$38 per acquired voter. This is the wondrous thing about text-messaging: Studies show that text-based get-out-the-vote appeals win one voter for every 25 people contacted. That’s nearly as effective as door-canvassing, but it’s much, much cheaper. Text messages cost about 6 cents per contact—only $1.50 per new voter.
We can assume the calculation is similar when selling things other than political candidates as well.