Assumptions: Cost of the car is the total cost. This includes tax, fees, paperwork, registration, and basically anything to the one-time payment to purchasing a car. If you do not know how much those are, the cost of the car alone will probably be around $20-21k depending on where you live. The metrics that I have provided are from a 2012 Honda Civic which is about 10 months old. I did not include any other maintenance costs at this time but given a 10 year period, this cost should be negligible. I used 10 years because I figure almost all car models should be last at least 10 years. Calculating months, I assume month is approximately 30.42 days or (365 days / 12 months).
Estimate corrections: The car is newer so will be slightly more gas efficient than older cars. Gas efficiency is mix between regular mode and gas efficient mode. 80% of trips are within 15-30 miles; 20% 120 miles; about 2-4 trips over 500 miles.
Conclusion: The metro is approximately $2.50 / trip or $112 / month (45 trips to break even) or about $1350 / year, so commuting would save approximately $4000 / year. Living expense is much cheaper outside the city. On rent alone, I save approximately $800 / month or $9600 / year. Also, I expect my car to last more the 10 years (hopefully, 20 before any major problems occur) so effectively the final cost will be $6.58 less per day after 10 years. If you expect about $1000 in maintenance, then add approximately $2.80 per day.
Metrics:
Car: $24,000
Expected life: 10 years (~365 * 10)
Car per day: $6.575
Insurance (comprehensive): $1600 / year
Insurance per day: $4.38
Miles per day: 35 miles / day
Gas efficiency: 35 miles / gallon
Gas usage: 1 gallon / day
Cost of gas: $3.50 / gallon
Cost of gas per day: $3.50 / day
Total cost / day: $14.45 / day
Total cost / month: $439.52 / month
Total cost / month: $439.52 / month
Total cost / year: $5,274.25 / year
Costs do not include repairs, oil changes, and other maintenance costs.