The Ribbon Reefs are certainly much better diving than the so-called "Outer Reef" closer to Cairns. (Aside from maybe getting to see some different critters, I'd say pretty much any of the usual Caribbean dive spots provide better diving than what I saw of the Outer Reef.) Personally I also would opt for a Coral Sea/Ribbon itinerary if you're not doing the whole week on one of those more expensive boats, as I wasn't I wasn't too impressed by Cod Hole. (If you do one of the half itineraries, you are paying for the flight out or back as part of that trip cost.) Unfortunately I don't think there's a more economical option that gets you to the better stuff, at least not anymore.
Are you going to be in Australia anyway, or are you planning a trip there with the goal of diving the GBR? If you're going there there anyway, and a budget liveaboard is what works for you cost or schedule-wise, maybe it's worth doing as long as you understand what you're getting (or not getting.) But if you're planning a trip to Australia specifically to see some of what the GBR is famous for, like a bucket list thing, the cheaper outer reef trips aren't going to get you that and you will likely be disappointed.
If you're price sensitive and looking for good diving, I'd ignore the GBR marketing and consider other destinations. For example the Philippines have plenty of great diving and everything is pretty economical once you're there, even for fairly nice places. You can get pretty good deals on airfare sometimes too, especially if you catch a sale on Philippines Air.