Anytime I put more than $1,000 into repairs on a used car, I regretted it.
I have a used car. if I put $1,000 into repairs for it and get 12 months out of it. I win.
Can I lease or buy an equivalent car for $1000/yr, $83 a month? Usually not.
If I could, how much would the value deprecate in a year? More than $1,000? Most likely
$1,000 into a used car can be the wisest thing. I pretty much drive my cars till I hit 250,000 miles. It takes another $4k to usually go from 200k-250k usually because timing belt, valve cover and expensive routine maintenance comes up. But $4k to drive 50k miles? Totally worth it.
So you have to try to figure out, is this car still solid except for the thing that needs $1000 to fix, or is it starting to fall off a metaphorical cliff? And you never know. But sometimes the car feels less and less solid, and you can tell that it's time to get out.
Is that time the first $1000 repair? Not necessarily. (I've spent $1000 on the 100,000 mile maintenance on a Honda Odyssey twice, and not regretted it. I've also spent closer to $3000 on an unexpected transmission rebuild, and I haven't regretted it... so far.) But it's not a completely stupid rule of thumb that, if most repairs are $200 or $500, the first $1000 repair is a time to at least seriously re-evaluate the vehicle.