Why do people charge per keyword?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  Eva Brown: When a website rank or derank in Google search engine or any other SE then it is due to that specific keyword. It is a keyword that has CPC high or low not whole website. It is the keyword which is the main reason to bring the website on the top of others. So, charging per keyword worth.
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  Jordan Pearce: Sometimes easier to build pricing packages around it when you have a 12 month contract
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  Denys Dubner: Our packages, in most cases, are built around the number Keyword Families equal to the number of target URLs. This doesn’t mean we don’t optimize sitewide. This means our key KPI is specific keywords families performance.
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  Monique Zavala Rice: Because the larger the site is the more there is to update when you need to up the anny and boost rankings for keywords. This may mean refocusing content, rewriting content, content creation because it doesn’t exist, link building, making sure the whole site is structured right etc (which May mean a rebuild etc). The larger the list of keywords and or pages the more work there is, and this is one way to control client expectations. There are many ways to control it but if you don’t want to get into hourly and you don’t want a client expecting you to do whatever it takes for their 200 keywords then you better have a system in place of some kind to control the madness.