I've got a roof rack with 5 bike trays. Yes it kills the mileage when there are more than 2 bikes up there but really its the only way to do it with that many bikes.

If you are only going to take along 2-3 bikes, a hitch and hitch rack is probably the way to go. It'll probably still be cheaper than getting a whole roof setup even if you have to get a hitch (~$200 online, most cars its simple bolt on).

I'm not a huge fan of trunk racks since they can scratch your paint, easy to steal, but if I were to get one/recommend one, it would be a Saris Bones RS (assuming it will fit).