In theory unlimited but (if you dig in the T&Cs you'll find that they expect this to be 500GB). We're still on rubbish ADSL--after lots of tweaking by the phone company we're up to about 4800 down and 812 up--distance to the exchange is our limiting factor. It's hard to quote a price for this because we have a bundled service--internet, landline phone, most calls all over Aus free plus some free international every month. For this we pay $109. Yeah, internet is stupid expensive for crap service in Aus.
Hopefully we'll have a fibre connection in the next few months--they pulled in the fibre past our front door in August but won't start hooking people up until the whole suburb has cable past their doors. I gather the delay, believe it or not, is some people with aerial cabling because they have to work with the electric company to work on that.
What would I do with blinding speeds, Dave? Download software in less than an hour or two, upload my videos to Youtube in less than six hours, that sort of thing. We currently have six internet connected devices in this house so the connection gets overloaded rather easily.