Specifically, the two factors determining how much work is involved are the size and complexity of your site.
That means a neat and tidy, professional 15-page brochure website with standard customization is going to cost less than a highly customized 50-page site with additional functionality like advanced image galleries or shopping cart capability.
In terms of actual money, this variation in size and scope means the cost could be less than £1,000 or more than £10,000.