champion510 is correct. If you have 100 visitors to your website and 1 buys something, you have a 1% conversion rate. It doesn't have anything to do with Yahoo or Google. Most small businesses struggle to get even 1% conversion rate. The best converting websites make sales to more than 20% of their visitors. Even a very small improvement in your conversion rate, from 1% to 1.3% is a 30% increase in business. Small companies can copy the winning strategies of the best converting websites without spending a lot of money.
Well, all things equal, you can have variations in conversions based on the network you are advertising on. The reason is the type of visitor. People searching via Yahoo in my experience are more 'buy' ready than on Google. But of course, that is a gross generalization. you need to test traffic on the big 3 for every offer as some work better on different engines.
I've always seen a slightly higher conversion rate in yahoo. There ctr's seem to be lower though, so it evens out. The cost per click is also lower than google as well. Also, as everyone else has said, it isn't solely dependent on the search engine for the conversion rate, it is 99% the content of the landing page. I like to think of this for the breakdown: keyword quality = # of impressions ad quality = click through rate landing page quality = conversion rate Aligning all these will ensure a successful campaign!