I think (and excuse me if I'm wrong) that the most people assess the repair cost in two ways.
- If I repair the car will I get the full value of the repair out of it in time/mileage before another big repair bill?
- If I repair the car will I get the full value of the repair out of it in time/mileage before some numpty crashes into it and writes it off for less than the value of the repair?
The trouble is that a car with a book value of e.g. £1000 and a failing clutch has the same book value as an identical car with a new clutch.
The clutch on my cdti has lasted over 80k in just under 4 years - so at £1000 for the whole lot (dmf included) was worth it. If the car had had major problems beyond repair in less than two years I would've been out of pocket on the deal. (in that time it's also needed belts, brakes, injectors, and all three fuel pumps on top of normal servicing--- P.S. and a steering rack)
Point 2 is actually irrelevant - no-one ever gets the true value of the vehicle paid out in an insurance write-off - and it's not something you can plan to mitigate really.