fall into the category of throwing good money after bad? The answer depends on a lot of things and is different for everyone.
Obviously, one major contributor is the cost of the repairs. If the cost is more that the car is worth, it makes no sense to fix it. If the repair costs are less than the value of the vehicle and you are handy, you can save the labor costs by doing the repairs yourself. If you are not handy and your car has high mileage, is very old, has been giving you a lot of trouble or is generally in poor condition, it simply it is not worth it to repair it.
If you do decide to replace your car, you then have to figure out what to do with it. You can trade it in. However, most car buying guides suggest that you are likely to get a better deal without a trade in. Also, the car dealer will probably not give you much for it. For the same reason, it may be hard to sell it privately. Most car buyers do not want to purchase a car if it needs repairs, particularly if it does not run.
You can sell it to a junk yard for a couple of hundred dollars for it. They will sell all the workable parts for a lot more money than you got. You can dismantle your car and sell the parts yourself. That will cost you a lot of time along with some money for advertising. Then you have to decide what to do if someone claims the part you sold them does not work. You also have the problem of disposing of the left over parts.
Another alternative is to simply donate your car to charity. If you donate car, you will get a tax deduction of the greater of $500 or the amount the charity sells your car for. Cars4Charities will gladly assist you with your car donation. Just call them at 866.448.3487 or go to http://www.cars4charities.org/.