Setting a budget
In an ideal world every project would have an unlimited budget and nothing would be impossible. In my experience a tight budget actually improves performance because it encourages innovation.
Consider the following to help set a total budget on any project.

1 Internal costs
How you account for your own time or those of internal resources if they are to be dedicated to the project
2 External costs
Consultants, research, designers, print / construction costs, Advertising and Public relations
3 Opportunity costs
One off costs particularly with a new brand where there is no historical finance model to follow.
4 Contingency
Always make an allowance for the unexpected. Depending on the size of the project I would tend to allow between 10-15%
Payment terms
If you are working on a fixed fee basis you may be asked to pay the fee in stages corresponding to a schedule. For smaller projects it is common to pay 50% on commission and the remainder on completion. the project proposal should be clear about how outside costs and expenses are to be billed and if there is any mark-up for administration. (10-20% is not uncommon)
If you undertake to pay promptly you may be able to negotiate a discount.