I agree, the OP's question is difficult to understand, but in the interest of sharing experiences and knowledge, I can share some things that have worked for me. My company builds web applications, not just web sites, so YMMV if you're doing more brochure-level sites.
A helpful answer to give when someone asks how much their app will cost is to frame the discussion in terms of price ranges you usually work in—give an honest assessment of what it could cost. For example, I often tell prospective clients that I can build their application for $25,000 or $250,000—it really just depends on the level of complexity and polish they're looking for. Take an everyday application like Gmail, for example. Obviously, Google has spent a lot of time and money making that application as polished as possible and adding enhanced user experience and features like labels, folders, calendar integration, etc. They've spent millions on it. But if I were to build an email application from scratch for a client, it could be built much more cheaply, as long as the client and I work together to tightly define features. An email program could cost x or it could cost 10x, depending on what the client wants to put into it. More money will yield more features and more polish.
By saying "it could cost x or 10x," I help establish for the client what a reasonable range is for the project, and I also help introduce them to the concept that in the end, their requirements are what will determine the final price. It's my job as the professional to help them identify the smart places to spend their money to ensure that we get the best software out of whatever they spend.