The comparison people get wrong
The instinctive test is repair cost against car value, and it is misleading. A $2,000 repair on a $4,000 car sounds absurd until you notice the alternative is $400 a month for five years. The right comparison is cost per remaining month of reliable use — against the same figure for whatever replaces it.
Questions worth answering honestly
How is the rest of the car? A strong body and interior with one failed component is a different proposition from a car where this is the third major repair this year. Do you know its history? A car you have maintained is a known quantity; a replacement is somebody else's unknown. And is this repair the last one, or the first of several?
When we will tell you not to repair it
If the repair approaches or exceeds what the car is worth and the rest of the vehicle is tired, we will say so — even though that means we do not get the job. Structural rust, a failed engine or transmission in an otherwise worn-out car, and flood damage are the usual cases where the honest answer is to stop spending.
The middle option
Sometimes the answer is a partial repair that makes the car safe and usable for another year while you plan the replacement properly, rather than deciding under pressure on the day it broke. We are happy to scope that if it is what suits you.