True that they are cheaper and 75 degrees is the minimum you want to keep a betta at otherwise they will get lathargic and may even die if it drops lower.
Sure it would be nice to live somewhere warm like you do, I sure wouldn't mind! But unfortunately I don't :/ And I don't know exactly where Newcastle, Australia is except that it's in Aussie lol so I can't exactly speculate on how the temperature is down there. But the other thing with those heaters is that they really aren't reliable and you basically can't tell when they have died or anything since they don't have the lights. They also don't turn off so they are constantly heating the tank but it fluctuates.
Half of deaths (okay well I probably exaggerate, but there are some deaths that) are caused by temperature fluctuations when a temp is kept at a steady temperature it can cause stress for the fish which can lead to so many other diseases. It's worth it to buy a good heater than you can change the temperature on and avoid having to buy obscene amounts of medicines and stuff, you see?
I'm sure my logic doesn't make much sense, it did in my head before I said it though >< Anywho, yeah....that's all for now :)