How do your structure your sale offers?

I work in an agile environment and I find it very hard to make good estimates and sale proposals for new projects. When I make a proposal based on a mockup I feel that it restrains too much the development process, when I don't make a mockup the client always feels the need to ask for continual changes to the UI that don't fit in the original time estimate. What would be a good way to structure an offer offering minimal leeway to the client, while keeping him happy with a good and precise outline of the features that will be delivered?