In reality Cunderdin just becomes a tick in the box for alternate planning, if for whatever reason a heavy was to go there majority of those off a long haul wont have the duty to return to Perth, then you have customs, Fuel, and 400 pax which would probably be bused back to Perth as you dont have the luxury of other airlines to get them back.
So how does Ascent make money out of this? Do airlines pay for the priviledge of using YCUN as a nominated alternate given they save on fuel uplift? Im sure you would rarely get a heavy landing their to nab some landing fees.
As for xwinds in Perth they can occur at any time day or night so what RFF facilities will be in place in YCUN for this possible scenario?