$4-5K per car is a matter of deleting a goodly part of the options list. Some sound systems come to almost that much, and strictly speaking, a hand-held transistor radio provides a similar basic function.
But then, I come from an era when an AM radio was an extra-cost option, and air conditioning was unheard of.
It all comes down to what one considers to be vital to continued existence. Getting from Point A to Point B at a less expensive per-mile cost is much more important than the most recent playlist as reported by MTV.
Or even tweeting from the convenience of the driver’s seat.
The 4-5K figured just seemed high to me to convert a car to natural or propane gas. I grew up driving junkers where a 4 or 8 track tape or an aftermarket FM adapter was a cool modification. A/C was a J.C. Whitney fan/defroster. :-)