Over the two years I've had Ruby2sday she's returned 5.3 l/100km. That figure is skewed by the fact I have the V-Strom for my city riding, so most of the ST's travel is on longer rides, which helps immensely. It's around 6.25 when it's mostly city riding.
I'd concur with the 500km per tank- that's what I was getting on the NZ RT. Cost-wise, the saving in consumption is offset by the much dearer fuel- around 10% more expensive.