Estimating Price & Potential Beforehand

Pretend I want to rank a site/business in organic seo only and for one keyword only.

Now if that keyword was something like "plastic surgery new york", that is obviously extremely profitable. As it gets 2,000 searches per month with an $8 CPC for adwords.

So if I was to rank for this I could charge a client the total number of visitors the site gets for that keyword, multiplied by the CPC. Then I could give him 50% off that, so I'm half the price of adwords.

However, what if I wanted to rank for a keyword like "dentist + small town" (say town has a population of 30,000). And the keyword "dentist + small town" and any variation of that gets 0 searches per month according to adwords, along with no suggested bid.

Surely a dentist would still see value in having his website appear first for queries like "dentist + small town", best dentist + small town", "small town + dentist", etc -- regardless of the fact that by search volume, ads, and competition it is essentially worthless.

So how would you price something like this?

Thanks

Do you think Dentist + small down or best dentist + small town worth a try? if so then set the price to be like $0.20-0.50/click, If there is no competition as if you set price of bids higher, your result will show first on search engine list.