It all depends.
There are some flat rate companies that charge by the size and scope of the project (i.e., 10 public webservers, 10 internal servers, 3 firewalls, 1 domain, 100 users, etc. = $X)
Then there are some companies that charge by T&M (time and materials). In which case, the longer the project goes on, the more it costs.
We tend to do fixed fee assessments because we know how long a project should take for a given size. But again, it all depends on the size of the environment. For example, Netflix (in the US anyway) maybe just one site, but may be powered by thousands of servers, a huge infrastructure footprint, etc.
Recommendation
Get a few quotes from some trusted vendors/consultants and pick the one that works best for your comfort level and budget.