The most expensive milk? Cat's Milk! Have you ever tried to milk a CAT? Not for the delicate minded!For grins, I googled Yak Milk Soap. Yup - there's Yak Milk Soap. Also reindeer, musk ox, water buffalo, camels. And LLAMAs!!! I'll need to get my hands on that one for sure!
If it can be milked... it has been put into soap.