I recently sold an old car of mine to a scrap dealer for £140, it would have been £160 if I had delivered it myself. The car in question had no rod tax or MOT certificate, although it was in working order. I think at most I could have sold it for $400, assuming it had tax and an mot, but to get it up to that standard would have cost a few hundred pound. In the end I think I made the right decision in scrapping as I wasn't expecting as much as £140 for it. What circumstances would lead to you scrapping a car?
I think that most people don't know they can sell their car to scrap. I've sold a few old cars and I've never considered selling them to scrap and at least one of them probably would have been worth more than what I sold it for. So nothing like compare the prices, see what you can get for selling it and how much you would get from the scrap dealer.
I have a 17 year old truck that still runs. At some point I will be put into the position of selling it - possibly for scrap. One of the best deals around is to use it as a trade in on a different vehicle. In order to lure you in to buying, a dealer will often give you a better offer than you would get by selling or scrapping it. This is only a good option if you are replacing a vehicle, but it is something that everyone should consider when disposing of a car. Some dealers want your business bad enough that they will even take trade ins on vehicles that are not in running condition.
I would say when the vehicle becomes an environmental hazard, is when it should be scrapped. When I say environmental hazard, I mean besides that its exhaust pipe is spewing out a regulated dosage of emissions. Also, when it's breaking down enough, on top of having pieces falling off, etc and seems unsafe to drive any further is a good idea to scrap it.
When the vehicle is past the point of safe operation and it would cost more than the benefits of repairing it, I would say it's time to scrap it and get something more useful out of the metal components of the car through recycling. Now, if there's a chance it could be fixed up as a classic, at least try that route first. The second option would be to save it for parts if you have somewhere to store it, and you might even check out a junkyard to see if they might want it for spare parts. For many vehicles, spare parts are becoming very very rare, so don't scrap it if you can salvage extremely valuable parts from it first.
I would say when the car is so old or there is so much wrong that it's not worth the repair costs, then it's time to scrap. I scrapped my first vehicle in college... but it was a car that was a '92 I believe...lol. I think I got $150.