I am hauling a few horses this next weekend and will charge 52 cents a mile.  However, I will count all miles to go pick them up and bring them back.  You basically want to get at least 50 cents for every mile you drive.  By charging $1 you cover your dead head back to where you are going.  One thing you could do is advertise in the Share a Ride Forum and see if anyone has a trailer that you could take back with you along the way and get double mileage and make more money on your trip.