It will help you, and me, enormously if you have a realistic idea of how much you intend to invest in your site. It's pointless if I quote you $10,000 if you can only afford $5,000; equally pointless to quote $10,000 if your site will cost $50,000 to develop properly.
That's why working out full details of all the aspects of your site is so important. It gives you a clear plan; and once you've given me an accurate picture of what your site will be like, and how you want it to function, I can give you an accurate quote.
That's also why it's important to map out what's necessary for site launch, and what can wait. The Web is a dynamic medium. A site can be built up to its final form in stages, allowing you to spread the pressure on your cashflow.
Next: Think ahead