Basically your options are as follows:
1. A day at the main Bali GT site (Batu Abah) which does hold some very large GTs (just saw photos of a recent October 2011 claim of a 60 kg fish there for example... I reckon that was an ambitious shout, but it was certainly 50 kgs). You should understand that it is a fickle site and can be really, really hard work for no return. There are also good jigging spots very nearby with Dogtooth, GT, Amberjack, Ruby Snapper etc).
I can probably get you a mates' rate day charter out there with a very good boat for Rp 4.9 million (about AUD 540). It is about 60-70 minutes sailing from the main Bali dock out to Batu Abah.
2. Crossing the straits to the much less fished Lombok sites. This is a bit of a longer boat ride (90 mins to 2 hrs depending on the seas) but there are a lot more fish. It is mostly GT popping there with fish 10 to 30 kgs typical, and the odd real monster. There are some well kept secret jigging sites especially for Amberjack but I have rarely caught much in the deep water over in Lombok.
Same boat charter would be more expensive - more like Rp 7 million (about AUD 770).
I am a bit tied up next week myself and will be fishing in Lombok for 2 days this weekend. However, I might be able to join you for either option and split the cost.