If this contractor gives you a price before ascertaining the necessary details, you’ll probably see that price quickly start to skyrocket.
Only after he’s asked lots of questions—some of which you probably haven't even considered yet—can he give you anything close to a detailed estimate. And even then, there are many smaller points, like, say, what kind of bathroom tiles to use, that may get worked out during the course of construction. These can also impact the final cost.
The same is true for custom web development. While we had lots of experience building e-commerce sites before we started working with the YMCA of Metro Chicago, it would have been naive and irresponsible of us to assume that their needs were equivalent to those of a company that sells spices or online study guides.
Be wary of anyone who is so eager to win your business that they throw out a firm price before trying to understand your specific needs. It's impossible to estimate an exact cost of something that has never been built before. Rather, a developer can give you an estimated range based on previous projects he has completed. The more information he has, the more precise that estimate can be, and the fewer surprises for you down the road.