Dear Friend, It is really tough to estimate a Web Site's price unless you review it. The Price depends mostly on Functionality of the Website and the HR (Human Resource) Time you need to make it happen. Technology is also a factor involved. For an example Cutting edge technology like AJAX can increase the price of the same functionality higer.
I will suggest you to review the Site of your clients choice.
Check the functionalities and estimate the time you need to acquire that functionality + Design. When you got the time add some more time with it. Because you never know what would happen if anything goes wrong.
So now you got the result, Time. Derive the Money you want for spending that much time. I wish you got the equation and found the result yourself. Wish you best of luck.