The cost isn't in the radio, you have a single chip which is a complete AM/FM radio - the cost is in making cars quiet enough from an EMI point of view - something which, IMO, they should have to do anyhow to comply with Part 15.
https://www.cbsnews.com/news/am-radio-congress-automakers/
I can find other references as well if you want. EMI in cars has been an issue for a long time, automakers however did do the needed work suppress the noise from spark plugs, I do not see this as any different.
Any idea if the law requires the redesign (i.e., requires that the AM radio meets some level of audio quality when operating in the vehicle) or if it just requires the AM radio to be present and (somewhat understandably) the manufacturer don’t want to include an AM radio if it consistently sounds bad?