This is a difficult one to answer. SEO packages vary from business to business and depend a lot on the industry and how competitive the market is. Monthly costs as well depend on the aggression of an SEO campaign and how quickly a business wants results. The level of investment will be reflected in the results. For example, £400/month compared to £8,000/month will put your business in very different places. SEO has the best return on investment when comparing all other marketing channels.