They may have deep pockets but it's getting ridiculous
"So looking at ComScore's numbers, Microsoft's Online Services Division has increased the number of U.S. searches more or less attributable to its efforts (Yahoo and Bing) by about 0.5 percent in the past quarter -- roughly 85 million searches more in June than it would've had if nothing had changed. And for this Microsoft spent $728 million? Even if Microsoft had 85 million more searches in April and May than they would've had otherwise, that still works out to about $3 a search."
"It happens a lot. Microsoft started reporting its online financial vagaries in fiscal year 2002, when the effort was known as MSN. Since then, Microsoft has admitted to a net loss of almost $9 billion."