Gooseybhai: Jetstar will most likley give you a voucher as a credit and this voucher can only be used once without any change given.
i.e. voucher for 200$ and the flight costs 100$, then they keep the $100.
They wont budge on that and will simply ignore any request to get that money back thats rightfully yours.
I wouldn't accept vouchers. They will also likely have an expiry date. Don't think vouchers should have expiry dates. Some of the best retailers don't have any expiry dates on their vouchers, so you can keep them for years. The retailer is making interest on that amount you have paid for the voucher anyway, while it isn't being spent.