You might also want to think about these things:
- ownership of the files themselves and the design/content- yours vs. theirs?
- website maintenance- who is to be responsible for updating the site; will you provide them small updates for little cost, or perhaps a monthly contract if content changes regularly? Will you provide them with a CMS to manage the site themselves?
- terms of payment- typically we require 50% upfront and then the final 50% upon completion of the site and a final approval meeting, but they will need to know where they stand- most companies aren't too happy with a per hour quote only- they want some "ceiling" placed on the project so they know what they're getting into, and they won't have too many surprises.
-status meetings or reports- I know clients like to know what's going on so they may require periodic reports as to how the site is developing, or meetings to see for themselves how the site is coming together. This takes some of your time away from designing, but is an integral part of the client/designer relationship. It's good to build this in to your proposal.
-detail as much as you possibly can about the website and what it will do versus what it won't do. The more detailed you are in the proposal, the more comfortable your clients will be with what they are getting, and the less room is allowed for changes or surprises later on. Always keep in mind that these are generally not website people and they have no earthly idea what goes in to making a website of that proportion. I recommend spelling everything out for them so they get a better understanding of the amount of work that's involved. And as someone else suggested, make sure you spell out the fact that anything above and beyond the scope of work outlined in the proposal will be priced separately and on a per-hour basis.
A word of caution on over-estimating your quote, however- while it's good to make sure you're covered, I'd caution you on becoming greedy or padding your quote too much. Especially in today's market when competition is at a relatively high rate. For example, one of our first clients was quoted $20,000 to develop her site- a very simple informational site with perhaps 10 pages of strictly HTML code- no PHP, nothing fancy at all. As a small business, this was absolutely out of the question for them and we were able to provide a great website for less than 10% of the competition's price. Obviously you get what you pay for, but there is a happy medium there and a delicate balance. Because you are doing this on your own (I think) you have the advantage of having very little to no overhead (as opposed to larger firms who have many other expenses which they have to compensate for) - so I would definitely take advantage of that in my pricing if I were in your shoes.
Hope that helps!
-Elizabeth