Ken,
The best way to obtain these costs is by detailed recordkeeping. Others can give you an estimate of costs but that is based on their experience, wages, equipment, efficiency, etc., not your situation.
Start keeping detailed records of each step in the process. How long does it take to prep these logs, mill them, stack the lumber, etc. divided by the board feet produced? What does your employee's time cost (including benefits, taxes, etc.)? What does it cost to operate your mill? Amortize the cost of the mill and other equipment you are using. Don't forget your facility costs, rent, taxes, utilities, etc. There are costs that can be accounted for on a monthly basis and those that are more appropriately figured on a board foot basis.
Basing your business decisions on someone else's costs of doing business may lead to failure. I suspect that .30 p/bf won't be enough to cover your expenses when you consider everything. That may change if you process enough volume but I doubt that donated yard trees will get you there. There are niche markets where it might work but if you know your actual costs of doing business, you'll feel more confident in the management decisions you'll have to make.
Good luck with your endeavor, the more activity in this area the better. It'll reduce landfill loads and benefit all of us.