Figures pls boomy - figures pls.
The figures change all the time that's why I said 40-50% cheaper. I can show you my recent search, parking from 01.06-08.06:
As you can see the difference between the two Flexi fares is 42%.
If you are willing to book non-changeable then it's "only" 35%.
Fares already include the Jetstar discount.
Note that during public holidays and major events the difference is usually even higher.
What mostly changed this year are the non-changeable fares. I used to book for $55-$65 and then upgrade for few more $$ to At Terminal.
Going back to the recent quote, it will cost $106 to upgrade (doesn't give that option with Jetstar discount).
The advantage with offsite companies is full flexibility and no restrictions. It doesn't matter what time you drop off and pick up your car. They don't even require credit card for booking so even if your trip changed and you forgot to cancel, no big deal. Also, if you need to extend the parking period once the car is already parked you can negotiate the price, something I'm not sure can do with MEL airport.
The disadvantage is the ride to/from airport usually takes extra 5-10 minutes.