Excavation costs depend on site issues, like access for equipment and if there is room to stockpile soil for reuse, the underlying geology and the type of foundation. That your line items are not all perfect is the balance of an estimate...because an estimate is an educated guess unless you pay for the legwork necessary to lock down bids for every line in that estimate. All contractors assume that they will miss high on some and low on others and that the balance is fair. $300 per square foot sounds reasonable for West Coast pricing...a little low for SF, a little high for Portland and Seattle. Verbal agreements when talking about spending over $1,000,000 for a home is NOT a good idea. True, contracts are only as good as the people signing them, but the contract spells out just how the relationship is to work, what each person in that relationship is responsible for and how. You may not need a contract with your husband...you really do with your contractor. I write contracts for a living...I'm a contractor. It is the description of my duties embedded in the title. I tell each and every one of my clients to read my contract and to have a lawyer read it as well. The road to hell is paved with good intentions...don't go there.