Why is the Shelter 501 mono so expensive?


Rainy day and I am chilling listening to records. I have a shelter 501mkII, my second, and I've listened to a few mono pressings that I really enjoy. I am wondering about a mono cartridge. In looking into this the Shelter 501 mono is almost twice the price of the similar stereo cart. Other than units sold I can't imagine that there is a lot of difference between the two. Any thoughts?
128x128davt
They think anyone who would want a mono cartridge will pay extra, I doubt that demand is driving up prices and I can't see any extra costs involved.
Post removed 
Possibly the 501 Mono versions are created to order by converting the standard stereo version to mono. This requires labor = money.