Chandrasekaran:
Within a given phase, and to some degree even across multiple phases, there is an expectation that a significant amount of work within multiple processes are happening at once and not serially.
So, you need to start the process of estimating costs but you know you need to determine your risks (and figure out how much effort will be needed for QA, and figure out your resource plan, and so on and so on ...) before you can nail down the budget so you have to do those, along with many other processes, at the same time.
I hope that helps.