There was a really good study I was given access to about a year ago (yes, that's a lifetime on the internet) that did some really good comparisons and stereotypes of users of different search engines, the value of that user, their likelihood to purchase (eCommerce study), etc. They also did some data on the paid ads. Essentially it boiled down to the fact that Google still had the best value, even at a higher CPC, but Yahoo was working.
Google - The masses, good return, honest ad system, most expensive CPC
Bing - Way too much traffic from windows features, xbox, their phones, etc implemented all wrong. Low CPC, but really low conversions, high bounce rate and very little honesty (ie: cost correction, misclicks, etc) in their ad system
Yahoo - Low cost, reasonable traffic, high conversions on non-purchases, lower conversions on purchases - they attributed this to the mean age of Yahoo users being much higher, less willing to use credit cards online
I wish I had the raw study still - instead I only have my notes from it. Was a good study, but things can definitely change so I'm not going to pretend the data is still relevant to today, especially with all the politics surrounding search engines as of late.
Ultimately, for me the numbers (up here in Canada) for searches in general still dictate most of where we spend our money:
PS: Let's just stop and think about the fact that Google was paying $300,000,000 a year to be the default in Firefox.
And they released the competitor browser that ultimately took top position from Firefox.
