Couldn't really say why people buy really expensive soaps but suspect it is for the same reasons as people buy expensive brushes, razors and strops. I am gonna guess it may be chasing the holy grail, a certain scent or just to use something the masses don't. Personally and like in most other things you quickly hit the point of diminishing return on money spent. For me that point would be after the cost of a Tabac or MWF refill puck. My baseline for a good lather starts with the cost point of an Arko or Palmolive stick. I like to play in that range to sample the variety of soaps being offered. So long as a soap is slipperier than snot and I don't gag on the scent I am happy.
Bob