it costs nothing if the keyword being targeted is not having several hundred thousand searches per month. if its has just 100 searches per month in google keyword planner then its easy to dominate. your website will come as first entry in google in few weeks. absolute cost increases depending on number of searches.
For More Information Please Refer: https://www.quora.com/How-much-does-it-cost-to-be-on-the-First-Page-of-a-Google-Search