Second that. This is the best tool I've found to locate pesky HD hogs. http://windirstat.info/ seems to be a similar, Windows-based utility.

Like websites for any type of client, this depends on so many factors it would be nearly impossible to propose even a ballpark price without additional information. What is the scale of the project? How many page types, templates, fragments, components, etc. will you be designing? From what, if any, foundation will you be designing? What is the scope of what you consider "designing a website?" etc.

I could see a reasonable answer to this question being as low as several thousand dollars for a local municipality's website, to several hundred thousand dollars for a large, federal agency website with hundreds of content types, a complex information architecture, and several stakeholders with whom you'll work.

Then there's the additional questions of how you price your work.

Sharing additional details and insight into these unknowns will help others much more helpfully provide you with some guidance.