What it's worth to the buyer isn't what it costs you
Their alternative is a $1,000–$1,500 install. That's the number they're comparing against.
A buyer looking at an older car who wants CarPlay has usually already priced the alternative: replace the head unit, four figures once labour is in, and a day without the car. So when they sit in one of yours and it's already there, they aren't valuing a cheap accessory — they're valuing a feature they'd otherwise have to go out and pay for. That gap between what the feature costs you and what it's worth to the person buying the car is the entire argument, and it's the buyer who brings the four-figure number with them, not you.
We're not going to hand you a resale-uplift figure. You price cars for a living and you'd check it against your own book anyway. Put one in a car, run it against a comparable one without, and you'll have a number that's actually yours.

