How can I prepare project financial proposal before business analysis phase

Question

How Can I prepare a project financial proposal before the business analysis phase of software development system, what is the best practice for prepare the initial plan for the project and how to estimate each phase's efforts and cost with the only high-level requirement? 

Especially that sometimes it is very hard to adjust the budget after getting approval on project proposal from client/sponsor?

Answer 1

I use a number of methods to overcome this challenge.
1. If the software is off-the-shelf, obtain quotes from 2 or 3 vendors.
2. Base estimates on previous projects that were similar in size and function.
3. Allow lots of contingency in your budget.
4. List the assumptions that you made in order to calculate the estimate.
5. Ensure that the risks to the budget are clearly articulated - ie there's a risk that the budget will increase once you know the final requirements.
And finally, ensure that the proposal states clearly that the estimates are just that - estimates - and are subject to change once the detailed requirements are finalized. This communication is vital so that expectations are set.
Hope that helps...