The (Perth Oz) Terminals are on opposite sides of the airfield.
The shuttle bus is a poor service, however there are always taxis outside the terminal, you do not need to rely on the shuttle, but you will pay a bit more as its about a ten minute taxi ride. This for sure would be less than paying the $100 for dropping your hire car at the wrong terminal.
There are a number of 'budget' car hire companies; google will sort you out.
