Not an apologist for the airlines at all, but, costs in WA are more expensive.
As far as QF is concerned, BNE-TSV can have up to 7 x 717 flights a day on weekends and quite a few mid week. TSV also has Dash8's and 737 throughout the day. That will reduce costs associated with staff throughout the airport including security, refueling, airline staff. Fuel prices are cheaper, and the list goes on. The costs associated with disrupted flights would be significantly increased, i.e. Higher accommodation costs (if there is any available) replacement aircraft costs, I suspect not oo many overflyers that can drop in to resolve the pax situation, they would have to fly from Perth if and when an aircraft is available.
Whether that should increase airfares as you have quoted I doubt it but there are certainly reasons why they should be more expensive.