Bear in mind that's a violation of the contract to sell miles even to a family member and that your dad runs the risk of losing all his miles.

However if you were just theoretically calculating a fair reimbursement price where you paid him, I would use the value at which UA carries the miles on their books, i.e. 1.51 cents. If on the other hand, it was your dad using your miles, I'd charge him at least 10cpm