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You could get some useful information related to your question via tools like the Google Adwords Keyword Tool [adwords.google.com] - if you enter relatively generic words and then sort by "Estimated Avg. CPC" you can get a feel for the markets commanding higher payments.
Microsoft's Keyword Services Platform has a lot of similar data too, and more ways to extract it, but unfortunately that's invite only at the moment and so you may not be able to access it unless you're an Adcenter advertiser and receive an invite.