Many factors affect the cost of a web site including:
Web site size
How large is the web site going to be? How many pages will comprise the web site? Will there be a product gallery with images? Is there a text or do you need help with writing one? Will there be a choice of language?
Graphics
Is there a company logo that can be used in the web site or will one need to be deigned? Is the company logo in electronic form? Are there digital images to be included in the web site or print images only that need to be scanned? Is a professional required to take digital photographs (e.g. of products)?

E-business
Will you need credit card processing for e-business? Is the product going to be available for immediate download (e.g. software, music) or does it require shipping? Will the web site be integrated into an existing Inventory Control System at a store location?
Contact Forms
What kind of forms will you need? Simple forms that can be found Contact Us sections in most web sites or more sophisticated ones that require additional personal information from users?
Additional Functions
Are there other functional requirements that need to be included such as password-protected areas for members or new member registration with username and password? Will there be a need for a forum design?