Paying folks to use its search engine is an idea Microsoft (NSDQ: MSFT) has flirted with for a long time. See this InfoWorld piece on it from back in 2005. Now WSJ is reporting that the company is set to announce a new variation on this theme, offering users cash back on purchases they make through the search engine. TechCrunch notes that the service is based on its acquisition of Jellyfish, a comparison shopper that provides users with cash back on purchases.

Cash back, of course, is used in all kinds of industries. Even Google (NSDQ: GOOG) has tried a variation on this model for its PayPal-competitor Google Checkout. Will it work? Getting users to switch their search behavior is tough, and it will be a challenge getting it up to scale. If we lived in an economist’s dream world, where every individual acted perfectly rational, then it might have a better chance.

• Microsoft Acquires Comparitive Shopping Engine Jellyfish.com