Which ones give you cheap, but yet good-converting, traffic?
I doubt there is such a thing if you're aiming for high-traffic keywords. Obviously, you're going to have to pay more to get into a high position and to receive more traffic-- In other words, for SEs like Overture, upwards of $5 p.c. will get you good placement.
The cheapest and more 'popular' PPC I have found is GoClick.com (with the highest bid being around $.97).
These search engines have cheaper bidded keywords, however, the CTR may not be as high. I would recommend requesting a 'Keyword Tool' (something like: inventory.overture.com) from the PPC SE to predict your CTR-rates.
The best way to find out what works is to try the top 10. Some might claim to know the top 10 but fact is, no two companies get the same results. What works on one, just doesn't work on the other. It's nothing special, just some products don't mesh well on some as they would on others.
Most 'keywords' such as web hosting will be dominated by large firms, who drive their listings to the top for both brand and cust-acquisition purposes. These top-ranked companies have spent considerable time optimizing their listings from a keyword and presentation stand-point. Try studying their listings for structure and you may be able to improve your conversation and click-thru rates.
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