It really depends on the market. My company is in education and the expense for Adwords is astronomical. Yahoo gives us 2-3x the amount of daily leads that Adwords gives us, and it costs basically the same...however, we have to appear everywhere, so that includes Adwords, Y Search Marketing and Bing. One thing I did notice is that these PPC programs perform differently...just like how Yahoo converts sales on such a higher degree than Google does...not to mention, the keywords that work well on Yahoo never worked well on Adwords, and vice-versa.