but it is so prohibitively expensive that very few people are running it.
Well aside from human resources you can easily find implementations of unix that will cost you nothing (most common examples are BSD and linux). Besides - what is unix other than posix and few standard apps put together. Posix is something nearly all alternative
operating systems support - they all suck posix application juice. If you don't have IIS you need posix to run apache/lighttpd/whatever - not unix.