My cost to replace parts is not merited based the book value of my 2006 small SUV.
Why? The way I look at it the cost of replacing parts should be guaged against the cost of buying a replacement car. I'd look at cost to drive X number of miles comparing repair against replacement.
Of course there are a lot of variables that are difficult to predict. The current car, Hyundai, Honda? Mileage. Expected service life. Other replacement/repair costs which have not yet reared their ugly head.
If you have an 06 Kia with 290K on the clock you might be sensible to replace it. If you've got an 06 Carolla with 290K, you've got at least another 100K of driving - not w/o some future repairs, of course. It might cost less to repair -even though book value doesn't justify it- than to replace.
Unfortunately it's kind of an inexaxct science. And there is the hassle of breakdowns and the car in the shop.
My wife has an 04 CRV with 111K on it. KBB says it's worth less than $3K I woulldn't hesitate to put $3K into it rather than buy a replacement. The likelihood is I can drive it another 200K cheaper than I can drive 200k in a replacement vehicle. But, as I say, it's an inexact science. JMO but "book value" just has no place in calculating future transportation costs.