It's a valid question - one that is asked of me every time I discuss web projects.  Usually I answer the question with a question (something I really hate to do)... "What do you want on your site?"

What needs to be considered is your content (what people see)

Do you have a lot of graphics?  Do those graphics have to be re-sized and web optimized?

Do you have all your copy in electronic form, ready to be pasted into the web page?

Do you need scripting?

How many sections will you have?  How many pages?

These are just a few considerations that must be determined before a quote can be given.  Knowing the scope of your project and what your needs are is important.