Buy another car or continue to repair old one..?

I have a 1986 Volvo wagon with 210,000 miles, a 1994 Honda Civic, and a 1998 Pontiac Mini van both with 150,000 miles. They are all paid for and run fine but now they are starting to cost more and more each year for repairs to keep them running (a recent $1300 bill got my attention!). At what point do I stop pouring money into the old cars and buy new (or newer) ones? I don’t want to wait till the cars die and my family is stranded along the road, but I also don’t want to spend the money on a new car until it makes sense. Is there a formula to calculate when the repair costs are too much or some other way to quantify this problem? Also, when the time comes, is it better to buy new or used?

What was the $1300 for?
A good calculation for figuring out when to buy another car is figure out how much you think you can afford a month on car payment. Save your receipts, and at the end of the year, total out how much you have spent on each car, then divide by 12(months).
So, if you feel $300 is a fair amount, you’d need to spend at least $3,600 in a year on one of the cars to break even on that point.

oh, and for the new/used debate. Since you seem to keep your cars for a long time, why not buy a new car and keep it as long as you have the others.