I have both, an 09 800r gade and a 07 500ss. Riding with the girlfriend (500ss) on trails at that 30-50mph pace, we are just about dead even on gas and oil. We generally get 16-17 mpg on trails. Sometimes I'm better by a hair, other times she is. I think thats weather dependent (I have DPM) and if I "play" with the throttle while falling back. Oil is the exact same, which is 40:1. If your burning more than this then your not adjusted right. Don't blame the sled if it uses more, blame the mechanic.
On a side note, the 500ss used to get 18 mpg, but the 07 and newer have a lean midrange. I fattened that up by shimming the neddles .020:, fuel screw out another half turn, run 10 for plugs instead of 9's, and run premium fuel.
Again, these numbers are trail riding at a cruising pace, normally with a group. Number decrease when run faster, off trail etc, but you asked for the 30-50 mph range.