Thank you for using justanswer. Since having the employee live on your ranch would be for your convenience, the Wages not paid in money.
for the room and board would not be subject to any tax withholding.
You may basically offer as cash wages the going rate in your area, and you're correct that the value of the room and board should absolutely be considered when determining the amount of cash wages that you will be willing to pay. You must at least meet the Federal minimum wage of $7.25/hr, but judging from the wording of your question, I think that really isn't much of a problem.
To some extent, the value of the room and board is subjective, depending on the type of lodging, general condition of the building, etc.
Your withholding requirements for Federal, state, and all payroll taxes
will be on the cash wages only, not the value of the room and board, again, since this would fall under the "for the employer's benefit on the employer's premises, and as a condition of employment".
Please see below:Publication 15 (2009), (Circular E), Employer's Tax Guide
I hope this helps.