Are you going to do this often or just once? Air drag is a considerable expense. If you can enclose the bikes in a somewhat aerodynamic container, you would win. Having the bikes behind the car is better than on the roof. I once took a vacation with four bikes on my car's roof, and the noise and drag was tremendous. The car was a lot slower and got much lower fuel efficiency. Are you trying to optimize cost or just space?

I was able to put seven bikes inside a VW Passat station wagon by taking most of them apart to various degrees. Removing handlebars, pedals, and saddles helps, and if you do this, you can get a smaller container than if the bikes are intact.