Those were the first words out of my mouth as well Don.
Like myself, you are also a logical thinker with a degree if mechanical savvy.
Not only the starter motors. But the batteries on stop/start vehicles can cost £300+ instead of £100 ish.
Mrs DD's Merc had only done a handful of miles in the year prior to my purchase. So part of the negotiations included the dealership fitting a new battery.
From memory it was over £400
It's just a very high capacity Varta but it wears a Merc badge. Hence the even more crazy price.