High competition keywords (i.e. keywords that many people are searching for and that many sites are trying to be found for) will be more expensive and lower competition terms will likely cost less.
How much it costs to run an ad or promote your search results will depend primarily on how much competition there is for your keywords. High competition keywords (i.e. keywords that many people are searching for and that many sites are trying to be found for) will be more expensive and lower competition terms will likely cost less.