I love fuel octane threads
High octane fuel doesn't have any more energy than lower octane fuel does, it's simply more resistant to auto-ignition (i.e. detonation.) While the energy from a spark is enough to light off both low and high octane fuel mixtures, the energy found in the conditions in the far reaches of the chamber during a combustion cycle -- pressure & heat -- isn't enough (all else being equal) to cause higher octane fuels to autoignite; they need the advancing flame front to complete combustion.
If your engine and tune don't produce conditions conducive to detonation under operating conditions where it's most likely then running a high octane fuel is not required. With a relatively low 10.7:1 compression ratio (a CBR600RR has a ratio of 12.2:1...) and what I assume to be a fairly mild tune (relatively rich and a non-aggressive timing curve) the 500 is not a detonation-happy machine.
The manual for the 500 calls for fuel with a RON of 90. MON is usually somewhat lower -- 3, 4 or 5 points numerically -- so averaging them (R+M/2) to get an AKI in the 87-range means you can run "regular".
I tend to agree with what was said earlier about ethanol. Try to find a supplier of ethanol-free gas and worry less about octane. Side story: I have a thing for small gas engines and so seeing lawnmowers at the curb on garbage day elicits a sympathy reaction; if I see one I'm likely to take it to see if it can be fixed (and then sold...) My neighbor had his by the curb so I took it before the scrap-metal scavengers got it.)
Got it into my garage. Oil...check. Fuel in the tank...check. Compression...check. Tried to start it, nothing. It had spark so I figure it wasn't getting fuel. I took the carb off and sure enough nothing came out. Took it apart and found the float bowl caked in a white, flakey residue. The float itself didn't move (it was stuck closed) because of this junk. I cleaned the worst I could out and shot cleaner into the jets and air bleeds, pulled the float off and cleaned the pivot and needle (etc etc), put it back together and it fired right up. I wheeled it back to my neighbor and let him know he didn't need to spent hundreds on a new one and suggested he store the thing for winter with a stabilizer and run ethanol-free gasoline if at all possible.
I don't want that stuff in my bike so in my FZ8 I generally run Shell "V-Power Nitro+" 91 which, in Canada at least, does not contain ethanol.