Different prices are also charged by different company but what you have requirement and competition in Google. After analyse the website, SEO companies will charge the prices for SEO of your website.
Each and individual SEO projects having own specific requirements. First of all, you will have to do analysis of website concerning what's they current backlinks, traffic rank, page rank, Social Media presence, Keywords ranking, competitors analysis and much more. Then only you can decide that what will be the way to deal with project along with what cost and time frame to deliver.