One would argue that $5k for a trip in a Citation X from Sydney to Melbourne was the full market rate!
One would assume that the price would have to be for the whole aircraft as a charter figure versus per seat which would require RPT arrangements.
The system itself as in "EmptyJets . com . au" is a great idea, trouble free and merely links the end-user with the provider of the flight, a little bit like Angel Flight. Unfortunately there's no way to guarantee that the flight in any way represents good value. I hope companies don't try to make market rates out of what started as an empty flight with a view to backloading.