Budgeting processes and techniques

Custom racing aircraft are the product of the Acme Flying Machine Company. It designs and produces these for professional race teams. They develop and build these custom designs as single individualized units or for the ultra-rich in lots of up to 4 units. The orders are generated by the marketing and sales department, with help from the owners who participate in the management of the company. Each new order is assigned to a project team, which starts by making an estimate to the customer before a contract is signed. The project teams have a great deal of experience in these projects.

As would be expected, the owners are hands-on and have opinions on the estimates. The marketing folks also provide suggestions on what the customer is willing to pay.

What are some processes and techniques that you would suggest to make the budgeting process work well? Start with more general but also provide some specifics as related to creating detailed project budgets. Be sure to justify why you think your recommendations will work?