A search on flightcar shows results allowing 90 miles/day. At $28/day ($10+$18 in parking savings), thats $.31/mile. At $18/day ($10+$8 parking) thats $.20/mile. This is probably the worst case. Most people wont drive 90 miles everyday.
The federal reimbursment rate is $.55/mile. You have to pull gas reimbursment out of that because like normal rental cars (I assume) flightcar renters pay for their own gas.
A conservative estimate based on 20 miles/gallon and $4/gallon is $.20/mile in gas cost. Each mile the renter drives using your vehicle thus costs you $.35 in wear and tear and depreciation.
An alternative data point would be car lease mileage overage fees. They are commonly $.15-$.25/mile and are supposed to represent the extra depreciation of the vehicle.
So its in the right neighborhood of value to the car owner but probably doesnt represent much profit and may not be worth the hassle.