I just went through this with a client. I had to think about what was said in the discovery meeting and then also think about how the client conducts business and where their revenue comes from. I went through a few bullet point lists that gave me some priorities for the business and a few options for how a customer might go through a process of paying for the services being offered.
Instead of mocking up working prototypes, I spent a little bit of time in Illustrator making a few static options and pairing the static options with a bullet point of what each example will do. Each example was another increase in complexity, and I tried to make each jump significant enough that it wouldn't allow them to merge the options or allow feature creep.
As far as a specific dollar amount quote, double whatever you think the quote will be. If you come in under the budget, your client will feel like they got a good deal when your final invoice is less that originally estimated. And if the vaugue-ness continues through-out the project, you won't feel gipped. I would recommend setting the project with very specific milestones that the client has to sign-off on before you proceed to the next milestone. It'll, again, prevent the client from backtracking on you within the project. Your milestones can be something like achieving each bullet point that you made earlier.