I've been tracking my costs since I purchased my car in October of 2011. I'd like to present this information so that anyone looking at purchasing a used 2.0L, GD chassis WRX will be able to have anecdotal evidence to prepare them for the costs of ownership.
In October 2011 I was lucky to find a single owner, unmodified, 2005 WRX with a full service history and about 75k Miles. I feel extremely fortunate to have found such a clean example of this year-model, and have not seen others like it for sale in my area.
I purchased the car with an extra set of wheels, a radar detector and an aftermarket head-unit. The car also came with the stock wheels and stereo. I paid $13,250, before tax and interest on the loan that I took out. Starting at purchase, I have documented all of the costs associated with operation and maintenance of my vehicle. Here are some plots which give an idea of cost per day, cost per mile, and fuel usage.
First (upper left) is a histogram of my fuel usage per mile. I track the number of miles that I've driven on my odometer and the number of gallons that I've used by the fuel pump at the gas station. Many of my miles are freeway miles, so my most-occurring fuel economy numbers are between 26-27 mpg.
Second (upper right), a similar plot shows the fuel used versus miles on the car. I've plotted lines for 25 mpg and 29 mpg for comparison. My average so far has been around 26 mpg.
I've also plotted my total cost of ownership per time and per mile. This includes fuel, insurance, maintenance, etc. The slope of these plots gives the cost per day and cost per mile (neglecting discontinuities due to insurance, maintenance, accident repair and purchase).
The lower left plot shows that my average operating cost is around 16.5-17 cents per mile. If I include all of the discontinuities in my calculation, my recent cost per mile is about $1.28 per mile (or about $0.55 per mile without initial purchase price).
When you are purchasing a vehicle, it may be important to consider the cost amortized over the total time of ownership. Thus I have also included a plot (lower right) of cost vs time. When I first purchased the vehicle, I only drove on the weekends (about 100 miles per week), thus my cost per day was around $3. When I moved and began to commute (about 300 miles per week), my cost per day increased to $10-$13 per day (not including discontinuities).
Notes about major discontinuities:
Insurance: First 6 mos. of full coverage was about $428 (AAA insurance), second 6 mos was about $434. The third 6 mos. was $630 (same coverage, no accidents or tickets, but I moved, Esurance). The fourth 6 months was about $1132 (same coverage, State Farm, after accident).
Accident and Repair: I had an accident on 2013-02-08 and spent about $4900 repairing the car until 2013-04-27.