For example the cost of a keyword in Google PPC system does not equal it's value, just how much people are willing to pay for it. Really not sure if the question makes sense, so please feel free to ask questions. Thanks!
The value of a keyword is related to how much money it brings in for your business.
In AdWords, typically you would discover this by running ads for various keywords and using a tool like Google Analytics to discover how many users coming in from an ad on that keyword actually buy something. From there you factor in your margin on the sale and the cost of advertising on that keyword to determine if it is profitable or not. Something similar could be done for "natural" SEO as well by looking at sales vs. which keywords a person searched for to find your site.