It has nothing to do with material costs at all. It has to do with the fact that if the subscription price is to low then no one will purchase it up front and everyone would go to a subscription.
Some have proposed a subscription rate of $150/month. At $150/month 67 people would be required to replace just 1 individual each month electing a subscription over paying the full amount. At that rate Tesla would lose revenue compare to today.
In order to continue a strong stock value Tesla must continue to show a strong revenue growth. Having FSD selling for a price that would generate an overall lower revenue would therefore lower Teslas overall growth.
I think you’re missing the point of subscription. No matter the price, it’s an ongoing income for the life of the company. It creates stability, that is more valuable then a large chunk of money at once.
It has also been proven in the last few years that there is more money to be made with subscription based ownership of things. It looks like less money, but the math works in the benefit of the service providers. In this case Tesla.
Imagine Tesla was able to have a consistent revenue stream coming in, and not depending on car sales or solar sales and etc. This is way better for the bottom line the bad times, and a boost to the prosperous times.
People who understand how to make the most of their money may choose to do subscription knowing that their investments of the price of FSD in certain stocks will more then pay for it. Others have enough money that paying full price makes little difference to their lifestyle and will find that more convenient.