Food expenses (for the "board" part) are easy. Take the average of what you spend for food in one month to feed a household of four, and divide it by four. That's what it costs to feed one mouth in a household of four each month.
For the rental portion, there is no magic formula. A room that would rent for $500/mo in Wisconsin, would rent for $2000/mo in Hawaii or in California.
If you were renting the room to a non-family member, what would you expect them to pay? What do you believe is the maximum they would be willing to pay?
Also keep in mind that if you are claiming 529 funds were used to pay "you" for room and board, then you're expected to report that income on your tax return. Since you're renting the room to your son, it would be reported on SCH E as a part of your personal 1040 tax return. It would also behoove you to have a written and legally enforceable contract, just in case of audit. The fact that you are renting to a family member has the potential to draw the attention of the IRS, because if you are renting to a family member at less than FMRV (Fair Market Rental Value) then those rental expenses that exceed your rental income are not allowed and can't be carried forward either. You just lose them, permanently and forever. That can bite big time in the future when you sell the property and have to recapture depreciation.
Finally, keep in mind that the room and board must be "in direct support" of the education. For example, if the student does not enroll for the summer semester, then any room and board paid for that semester with 529 funds is not "in direct support" of the education, and therefore is taxable income to the named beneficiary whose SSN is on the 1099-Q.
