As an experienced and qualified Project Manager I can say you would be very surprised the amount of people who have no clue what they have spent or want to spend.
People can't balance their chequebooks let alone budget and plan for multi K project. I don't mean to be derogatory its just human nature.
Potential clients who invite me to tender sometimes say they can't afford my cost yet happily blow their budget and end up in legal wrangles with their builder.
Most people are too afraid to expect a fixed price from a builder and contract it as such.
A comprehensive list of expenses is hard to quantify without planning and daily management.