This has been addressed quite a few times, but it probably is a combination of not just cost, but also manufacturing time. The panel itself and the wiring are the most likely things to need human installations, since they require working in smaller spaces. If it takes a couple/few minutes to run the wire, then another couple to install the screen, you're talking a significant amount of time that could cost you a few cars a day at Elon's stated "goal pace". So now instead of $87, you are talking thousands per day of lost profit, which they have to compare to the (admittedly subjective) opinion about the second screen being unnecessary. Not saying it's the right or wrong choice, but they have to make this decision for hundreds of different potential parts, and it's "death by a thousand cuts" when you are trying to get cost and efficiency down.