The proposal from analog-guy.com will include a time estimate for completion, a list of deliverables and a cost.  The cost is fixed unless I state otherwise.  It will stay fixed unless the scope of the project changes during the work period.  The time estimate is a good faith estimate of the time and is what I expect the job to take barring unforeseen problems (illness, communications interruptions, etc.).  

Usually the client does not pay a thing until the job is complete and the deliverables are ready to be sent.  Only on the occasion of very large jobs will interim payments be required.