Costs vary a lot for developers and designers, depending on the size and complexity of a site, the market for this expertise in a given region, the skill and experience of the developer or designer, and arbitrary aspects of pricing.
Follow one of the Golden Rules: do not spend a lot of money on an elaborate site that may not fit your business needs. Remember, the best sites cater to their customer's perceptions, not the desires of the designer or even the business owner.
Many companies offer full-scale strategic consulting with site development. They will offer traffic and marketing strategies that reflect both your strategic business needs and your site design. If done well, this can be an exceptional resource for you; if not, it can add significant costs with no return. If a developer or designer offers you these services, find out whether they are strong in strategic planning and e-Commerce marketing, not only the visual aspects of site design. Also, decide whether your company needs such expertise at this particular time.