Hah, I've been trying to figure out this same question and it seems there is no good answer. Everyone tells me to get an enclosed trailer for the Harley, but again looking at dropping 4k+ and another thing to store in the driveway.
Tow Dolly seems like a good solution for a car though - one thing I read about towing with four down is that it requires more money to rig up the vehicle for towing, and is usually specific to that vehicle. I could be wrong though.
That Dolly+Motorcycle rig is nice, but for 6k forget it! :)