My understanding is that the half pay thing is only a guarantee so the new employee's can receive some sort of salary for the period that the PNG Govt Work Permit is not issued.
Plenty of PX guys have been paid full salary from day one. That being a result of being given timely notice of a start date, and themselves being proactive in getting all the paperwork, history, medical information forward in a timely fashion.
Mach, PX does have a bonding system, with the value being based on the type. The figures that are mentioned seem quite fair really, as does the timeline the bond reduces over.