The fact that towing does add a major cost is a good counter point to the article. Patsy Z and I just went up to Atlanta in a Suzuki Swift rental and the cost of fuel for that was $120 total - round trip. So yes, fuel will be the largest impact on the cost. Unless you can find a way to split it even further.
Example: I am going to co-drive the Pebble which costs me $160 (about the price for a pretty prepped car) for the seat and then I can pay for some of the fuel bill (which is optional). If you and Charles co-drive and find two ladies to co-drive in SML then you can use that seat income for fuel/tires.
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if you can only race me in a straight line then whats the point 
...91 SMF SCCA
http://www.flickr.com/photos/8341770@N07/5601083432/http://www.flickr.com/photos/8341770@N07/5601083432/ by http://www.flickr.com/people/8341770@N07/, on Flickr
I see people out there spending money on the car to improve it … If you spend half that effort just working on your driving skills, making you and the car work together as a team, then you’ll go faster – period. To make the car go a second faster on the track takes a lot more energy and money than making yourself drive it one second faster.
– the late Tom Thrash SCCA Solo E Production