Any web pro worth their salt will know that such a question is literally impossible to answer without a good amount of additional information. Imagine going to an architect and saying, "I need you to build me a building. How much would that be?" Naturally, we would not expect them to be able to just rattle off a quote and then get to work.
Just a few of the questions I'd expect that architect to ask, which also hold true for websites, are:
- What kind of things do you want people to be able to do there? A car showroom and an office building are both places of business, but designed for very different interactions. The same is true for, say, an e-commerce site where the customer is actually placing orders for widgets and a piece of "brochureware" that is intended mainly to inform people of the services you provide, and the costs will likely be dramatically different.
- Are there particular materials that you want or need to use? Just as brick and steel will have different utility in different buildings (and have different costs), so will different technologies. Building a basic HTML and CSS website is a whole different ball of wax that implementing a content management system built on PHP.
- Do you have the know-how or resources to maintain complex systems behind the scenes? A fancy graywater recycling system would likely cause more problems than it solves if the owner fails to maintain it, and the same is true for many web technologies. Before you even start building your site, you should have a plan in place for its maintenance, and a good web designer will help you do that.
- How will people find your business? A restaurant near a highway, for example, is likely going to require a big, eye-catching sign near or on the building, intended to lure people through the doors. An office building in the same spot, however, might not need to draw such attention to itself since it doesn't rely on street traffic for business. Again, the same is true for websites: do you need to spend a good deal of time and resources optimizing your site for search engines, or integrating social media into your web presence? Or will your customers be driven to your site in some other way?
I could go on (and I'd love to hear your own suggestions for such questions in the comments), but the basic takeaway is this: for most businesses, a website, like the building within which you conduct your business, should not be an arbitrary decision or an afterthought: it should be an integral part of your overall business strategy. The best web professionals will do more than simply push pixels around the screen for you; they will help you articulate that strategy and figure out how to present it to the world.