Thanks for the comment. I try to make my proposals in detail - include designs of the app that we're going to build. I haven't tried the prototype approach - I might try it next time. Just to be clear, you develop the prototype for the client's app, right? You would need access to the client's APIs, etc. which you might not have in a sales phase. I think using mock data is another way around this,

Follow up question, when you develop this prototype - In cases where the app is not very big, doesn't the client think that almost 50% of the work is done and hence may try getting this done in cheap?