Still on its cashback kick, Microsoft buys comparison-shopping siteSMicrosoft will acquire Greenfield Online, which operates European comparison shopping site Ciao.com, for $17.50 per share or $486 million. Ciao works by directing shoppers to some 2,200 merchants, who then pay Ciao when those shoppers buy goods. ComScore says Ciao sees 26.5 million visitors a month. Last quarter, Greenfield reported revenues of $36 million and $2.1 million in profit. Following its strategy to split search marketing revenues with Web searchers, Microsoft says it eventually plans to share with them some of the money Ciao's merchants pay.Newspapers have stuffed their Sunday editions full of coupons for as long as we can remember, so there must be some money in being the middleman who passes on word of discounts and deals. The difference is that coupons are simple. They are delivered to you and then you just cut them out and hand them to the cashier when you buy your milk. Microsoft's cashback plan has a higher barrier to entry. We're skeptical as to how many searchers will take the time to give Microsoft access to their bank accounts in order to earn a couple pennies back on the book they just bought from Amazon.com.