The other issue is uncertainty - you have no idea how much you will earn on a given day or if this will be the day your car finally hits one pothole too many and dies.
(Income - expenses) / hours worked = hourly wage
Is your hourly wage reasonable? If so, it may make sense to optimize away your expenses and generally stay the course. If it's not, would changing your work hours or location make it reasonable?
Unfortunately, I get your feeling that people only look at the income and ignore the rest of the equation (hey, I got $50 tonight! I had to drive 100 miles though...). If you're making $3/trip, you had better drive less than 6 miles or you just lose.