TSE tends not to like opinion-based questions but his one has a bounty so I think can't be closed except perhaps by a mod. However, there are some facts that might help you decide.
Taking just Europcar as an example the cheapest rental I have found for an Australian resident, over 26 years old, noon to noon, 1 to 30 November to and from Perth City is AUD 1,081.07 for a Nissan Micra Manual. This includes mileage of 4,350 km per rental, air conditioning, Administration Fees, Licences & Fees, Airport Railway Station and GST.
Taking carsales as an example the cheapest vehicle I can see for sale there is $1,200 for a 2003 Daewoo Lanos SE Manual with 216,160 km on the clock, aircon and a free 12 month warranty.
The price excludes Government taxes which I think are around 5%.
So by the looks of it hiring would be cheaper unless you achieve at least $150 at resale plus however much you have to spend on a bought car that you would not have had to for a hired car.
You might be able to knock a little off the purchase price it you can negotiate away the one year warranty which presumably is next to no use to you. Doing so though risks a huge repair bill, though you could end up paying a huge mileage charge to the hire company if exceeding their allowance.
If either car needs attention during the month you are in Perth you are likely to be without wheels for longer if relying on a purchase rather than a hire.
You may also lose time in advertising (and perhaps cost) and demonstrating the car for sale.
My guess is that if $1,200 is reasonable value now it would sell fairly easily for $600 in a month, but perhaps not for any more than that. If you have to abandon it I do not know the consequences by way of any fines for dumping.
It looks as though, for low mileage, you might save around $450 – more for high mileage - by purchasing, but be driving a less fuel efficient and older car, with more inconvenience. Inconvenience could include paying the tax and registering the car in your name (subsequently transferring it) and if not normally resident in Australia sales proceeds achieved just before you depart the country might incur a conversion fee. Leave it in your name and you might return to Australia to some hefty speeding fines, etc.
I know what I would choose but, as mentioned, we are not here to offer just opinions.