AdWordsAdvisor advises about how to budget AdWords campaigns.
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One additional thought on this thread...
There is a (potentially) negative effect to leaving 'clinker' keywords in your keyword list: they can effect how often your ad appears. Some details:
AdWords suggests a daily budget sufficient to show your ad every time someone searches on each of your keywords. Essentially, the program looks at all of your keywords (including 'the clinkers'), then budgets for them.
If your actual budget is lower than this suggested budget, your ad will not appear every time.
However, you can lower the suggested budget by reducing the number of keywords that must be budgeted for. And most likely, the first keywords to go would be the 'clinkers'.
You can quickly see how your actual daily budget and the suggested daily budget compare. Start on the 'View/Edit Campaigns' page, then click the 'Edit Campaign Settings' link.
On the 'Edit Campaign Settings' page, if the suggested budget is higher than your actual budget, you will see a message to the right of your budget that says: "To maximize the number of clicks, budget $X.XX/day." This is an alert that your ad will not be showing for every search.
On the other hand, if you don't see that message, then your budget is sufficient for your ad to show every time.
This is worth checking from time to time, as the suggeseted budget can change depending on what you and your competitors are doing.
Whew! I have to shorten these posts! I have been called 'wordy' more than once in my life...