30+ years of advising start-up companies and emerging entrepreneurs, and more specifically handling the financial forecasting and modeling aspects of the deal, have shown me that the issues you mention---cost and capital requirements forecasting---are very problematic for the entrepreneur.
However, as a financial management advisor to so many companies in the same stage of the game as you, I've spent a great deal of my practice, and applied a great deal of my background and expertise, to helping my clients develop forecasting models with the sophistication and flexibility to allow them to answer all the "what ifs" regarding the cash flow projections, cost expectations, capital budget and financing requirements, and the like.
The models I build can be a simple or as intricate as the situation requires, and they're always custom-tailored to fit precisely the set of facts at hand.
I'd certainly enjoy a brief chat with you concerning your situation and needs, and then get started right away on applying all the details of your particular situation into a forecasting model that'll give you exactly the answers you and your investor need.
In any event, I wish you best of success with your venture!