This is a deceptive question. Getting top positions for your main keyword can be a varied expense dependent on the niche a business is in. A business that plucks chicken feathers from dead chickens would probably be a lot less than accident attorneys; not much competition for the former.
Also, many questionable SEO “experts” paste a city after the main keyword which to them is still somehow the company’s “main keyword” but it is not. Main keywords with city’s after them are usually useless with zero traffic stats.
So to answer the question….its impossible to answer without knowing what the keyword is and what the niche is. Plus it also depends on the methods you would propose….adwords vs raw ranking? Buying top level placements is only as good as the money you spend….some “main” keywords cost HUNDREDS of dollars per click. Spending money on search engine rankings may still not increase a company’s sales if other issues haven’t been addressed such as the quality of their site, the businesses location and appearance, etc. So unless you address the entire picture rankings are worthless.