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Do you mean ROI, having provided your users with a good experience, or regardless of doing so? In the latter category, for example, come advance-free-fraud, timeshare, fake drugs and fashion items, etc, and they can rake in millions.
Can you be a little clearer as to what you actually want?
[edited by: DamonHD at 11:49 pm (utc) on Feb. 13, 2007]
Sticky me if you want more details, but my firm (Efficient Frontier) is managing $300M+ in annual ppc spend worldwide (more than any other firm) and has dozens of clients for which we manage to Profit. We're applying portfolio trading algorithms from Wall St to the optimization of large ppc campaigns for ~70 of the top 500 ppc advertisers, and have been doing this for 4+ years now.
I'm obviously partial, but our solution does what you're looking for, and has the benefit of having been proven at scale with over 100 extremely large ppc advertisers.