From memory from when I looked, as soon as you add dimensions over what a normal car is the price grows astronomically, I think for interest's sake I looked and it was actually quite a bit cheaper to take two cars with a similar total length than it was to tow anything.
Edit, just had a look at some random dates and it looks like for us and our camper it would normally be about $1.8K for a daytime trip return and about $2.5K for the night time trip with bunks. Christmas time would be over $3K for the night time run.