Earlier quoted context omitted.
Yeah, where I am, it's about 5/3 as expensive. The upper bound improvement factor you can get over driving due to flying in a straight line is 2/sqrt(2), so your best case cost-equivalent efficiency is about 11 mpg.
This is why I'm excited about electric airplanes and driverless cars. A GA electric airplane has about 1/10th the energy cost of an avgas one. The cost of avgas is what discourages me from owning an airplane.
Even at 100 flight hours/year in a thirsty airplane, you'll spend maybe $10K on fuel. It would take a very simple airplane, hangared very cheaply, and no abnormal maintenance events for the year, for those other costs to be under $10K.
Few privately owned planes fly even 100 hours per year, and most of the non-fuel costs are largely fixed, meaning you pay them all just to fly the "first hour" and then subsequent hours just cost fuel and a small wear-related maintenance allocation.
Airplanes are cheap to buy, but not cheap to fly. The cost of gas hurts, but doesn't dominate in my experience (owning three airplanes over 15 years, and flying my family on most of our domestic travel east of the Mississippi).