Whats the price of 1 keyword in top of Google

Hello guys,
Im working in seo-company and every order we are thinking about how much will cost for our customer every keyword with position top10 in google.

Is exist formula of price for keyword? Something like 10$ for 1000 searches per month with no competition. But if competition is exist? If they are not so strong? How to calculate price of 1 keyword?