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If I try to get by cheap in one AdGroup and the keyword gets a low position and CTR, then the keyword gets disabled for any adgroup/campaign that it is in.
I learned the hard way that you're better off giving the keyword the bid it's worth and requires to position high. That way it will always beat out the competition and show.
I lost some good keywords by mismatching the bids (:
If you want to promote something that doesn't convert as well as something else in your portfolio, make a 2nd account and go play with that there then.. 2 ads.. same keyword.. 2 diff budgets and cost line items.
make a 2nd account
True the 2nd account won't hurt the keyword in your primary account, but read the AdWords TOS on that 2nd account finely. Second accounts are discouraged if the intent is simply to get a 2nd ad showing for the same or similar products for the same search.
In other words, if you set up a second account solely to double your exposure, they'll get you!
Correct me if I'm wrong, anyone, but that was my take on it.