When’s your current insurance due? You’ll probably find adding a second bike/scoot adds little if anything to the cost, the difference for my fireblade or blade + pan-euro was a £6 increase at renewal time. No doubt they will shaft you mid term though
The pay back on fuel alone may take some time, but you will also be reducing servicing/tyre/chain wear etc on the bigger bike, although I’m guessing a sv(650?) doesn’t have very wide tyres/chain nor chomp through them like a sports bike can.
I miss my CBF125 for MPG
I got on average 210 miles from a 9 litre refill. And that was done in inner London rush hour traffic with a wide open throttle away from the lights.