Simple economics mate - the miners will pay the bills for the hotels so the prices are set to what the market will bear.
(my gf used to do this job for Perth Adelaide and Melbourne for Holiday Inn, Crowne Plaza, so I know about this stuff).
Perth is an awesome place and it works a treat, as long as you don't try and do blatantly stupid things like travel on certain roads at certain times of day (or get a hotel room in the middle of town in the middle of the week). I love it.
Just because you can't afford to compete with BHP for a hotel room does not mean this town is rooted.
For cheaper accommodation try this
staywest.com.au/