I look at it from a more mathematical approach than most. I have a goal in mind on the cost per mile that I'd be happy with. Currently that number is 7.5 cents. That means if I buy a new accord for $21,000, then I have to get 280,000 miles to make that happen. That being said, sometimes, I'll find that a car hits a sweet spot in the middle somewhere when I can sell it and still get the 7.5 cents per mile. For example, if I buy a used accord for $16,000 and drive it for 50,000 miles, and then sell it for $12,500, because I got a great deal when I bought it and sold it for a good price on the back end, I can actually exceed my goal and not have to worry about repairs down the road by running my car until it dies.
In my opinion, you just have to keep an eye on it and sell it early when it makes sense. If you're driving a 2-3K car, you're probably going to do even better on a cost per mile, but your repairs might add to it a little more.