When we were car shopping, I developed a spreadsheet to try to figure out true comparable costs of buying + owning a car. In the vehicle category we were looking at (the very non-mustachian CUV), I was very surprised to discover that the vehicles were not depreciating quickly enough to have a used vehicle make financial sense, given the amount that we drive and the length of time we plan to own the car.
We live in a rural location and put about 30 000km on our vehicle in a year (I know, it's too much). We expect any vehicle to last us until at least 300 000km (so 10 years old). So with my calculation, I divided the cost of the vehicle over the number of years we could expect to own it (i.e., if a vehicle had 90 000km on it, we could expect to reliably own it for 7 years) and when I compared that to the cost of new vehicles, with the cash discount I was able to negotiate, the used cars ended up costing at least the same amount per year of ownership, with the added irritation that I would have to buy another car more often. This may not be true for all categories, but it's what I was finding for the highly-rated, 'fuel-efficient' and reliable CUVs I was considering. I also had an additional section in my spreadsheet for estimated gas costs, to make sure I wasn't looking at false economies of a much cheaper vehicle that ended up being a gas guzzler. If you drive very little, then this type of calculation doesn't make a lot of sense because your operating costs will be much lower and your car will last a lot longer than 10 years (and frankly, I'm hoping mine also will!).
Here's the basics of what I did - you can obviously sub miles but you'd have to calculate your fuel costs differently.
((expected total life km of the car) - (current km on car))/km driven per year = predicted number of years you own vehicle
total purchase price of car/predicted number of years you own vehicle = cost of vehicle purchase per year
((km driven per year)/100) x (predicted combined L/100 km) x estimated cost per L of fuel = estimated annual fuel cost
Sample calculation with dummy numbers on a vehicle costing $16 000 with 60 000km on it, and a combined fuel efficiency of 10L/100km.
(300 000 km - 60 000 km)/30 000km = 8 years
$16 000/8 years = $2 000
(30 000 km/100) x 10L x $1.00 = $ 3 000
Predicted annual cost of owning vehicle (not including insurance and maintenance) - $5 000
So your cost per year of owning the vehicle is the amortized cost of the purchase per year plus the estimated fuel cost per year. Obviously this doesn't take into account every variable, like insurance (we found insurance prices were pretty comparable on all vehicles in this category, and the age of the vehicle did not make a significant different), and recognizing the fallibility of predicting gas prices over the next decade (with mileage like ours, fuel efficiency became a very clear priority).
I found this was helpful in comparing used (and new) vehicles with various mileages, fuel economies, etc.
I know mine is not a mustachian vehicle purchase and that I should live near my work and cycle all the time, but just wanted to bring up a way to compare costs of vehicles in a way that reflects known habits and costs. As fantastic as our 2005 Matrix has been, we were getting to the point where it just wasn't fitting our needs as well as it used to (basically the kids got bigger and we could no longer stash things in and around their feet, so any sort of excursion became miserable for all involved as the car was packed to the gills). :p
I'm sure someone will point out some massively erroneous assumption I've made, but I was honestly shocked at how little the vehicles were depreciating and how the lost mileage/warranty/future life of the car was really not reflected in the purchase prices of the used vehicles I was considering in this category.
@meerkat - A hatchback is fabulously awesome with two carseats. Our Matrix served our needs excellently for the first 8 years of parenthood. We've upgraded to a Mazda CX-5 to hopefully get us through the next 10+ years. :)